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Data Structure - Binary Search Tree

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A Binary Search Tree (BST) is a tree in which all the nodes follow the below-mentioned properties −

  • The left sub-tree of a node has a key less than or equal to its parent node's key.

  • The right sub-tree of a node has a key greater than or equal to its parent node's key.

Thus, BST divides all its sub-trees into two segments; the left sub-tree and the right sub-tree and can be defined as −

left_subtree (keys) ≤ node (key) ≤ right_subtree (keys)

Representation

BST is a collection of nodes arranged in a way where they maintain BST properties. Each node has a key and an associated value. While searching, the desired key is compared to the keys in BST and if found, the associated value is retrieved.

Following is a pictorial representation of BST −

Tree Traversal

We observe that the root node key (27) has all less-valued keys on the left sub-tree and the higher valued keys on the right sub-tree.

Basic Operations

Following are the basic operations of a tree −

  • Search − Searches an element in a tree.

  • Insert − Inserts an element in a tree.

  • Pre-order Traversal − Traverses a tree in a pre-order manner.

  • In-order Traversal − Traverses a tree in an in-order manner.

  • Post-order Traversal − Traverses a tree in a post-order manner.

Defining a Node

Define a node that stores some data, and references to its left and right child nodes.

struct node {
   int data;
   struct node *leftChild;
   struct node *rightChild;
};

Search Operation

Whenever an element is to be searched, start searching from the root node. Then if the data is less than the key value, search for the element in the left subtree. Otherwise, search for the element in the right subtree. Follow the same algorithm for each node.

Algorithm

1. START
2. Check whether the tree is empty or not
3. If the tree is empty, search is not possible
4. Otherwise, first search the root of the tree.
5. If the key does not match with the value in the root, search its subtrees.
6. If the value of the key is less than the root value, search the left subtree
7. If the value of the key is greater than the root value, search the right subtree.
8. If the key is not found in the tree, return unsuccessful search.
9. END

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;
   
   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;
         
         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;
            
            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
struct node* search(int data){
   struct node *current = root;
   printf("\nVisiting elements: ");
   while(current->data != data) {
      if(current != NULL) {
         printf("%d ",current->data);
         
         //go to left tree
         if(current->data > data) {
            current = current->leftChild;
         }//else go to right tree
         else {
            current = current->rightChild;
         }
         
         //not found
         if(current == NULL) {
            return NULL;
         }
      }
   }
   return current;
}
void printTree(struct node* Node){
   if(Node == NULL)
      return;
   printTree(Node->leftChild);
   printf(" --%d", Node->data);
   printTree(Node->rightChild);
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printTree(root);
   struct node* k;
   k = search(35);
   if(k != NULL)
      printf("\nElement %d found", k->data);
   else
      printf("\nElement not found");
   return 0;
}

Output

Insertion done
 --15 --20 --35 --50 --55 --65 --90
Visiting elements: 55 20 50 
Element 35 found

#include <stdio.h>
#include <stdlib.h>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;
   
   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;
         
         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;
            
            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
struct node* search(int data){
   struct node *current = root;
   printf("\nVisiting elements: ");
   while(current->data != data) {
      if(current != NULL) {
         printf("%d ",current->data);
         
         //go to left tree
         if(current->data > data) {
            current = current->leftChild;
         }//else go to right tree
         else {
            current = current->rightChild;
         }
         
         //not found
         if(current == NULL) {
            return NULL;
         }
      }
   }
   return current;
}
void printTree(struct node* Node){
   if(Node == NULL)
      return;
   printTree(Node->leftChild);
   printf(" --%d", Node->data);
   printTree(Node->rightChild);
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printTree(root);
   struct node* k;
   k = search(35);
   if(k != NULL)
      printf("\nElement %d found", k->data);
   else
      printf("\nElement not found");
   return 0;
}

Output

Insertion done
 --15 --20 --35 --50 --55 --65 --90
Visiting elements: 55 20 50 
Element 35 found

import java.util.Scanner;
class BSTNode {
   BSTNode left, right;
   int data;
   public BSTNode(int n) {
      left = null;
      right = null;
      data = n;
   }
}
public class BST {
   static BSTNode root;
   public BST() {
      root = null;
   }
   private BSTNode insert(BSTNode node, int data) {
      if(node == null)
         node = new BSTNode(data);
      else {
         if(data <= node.data)
            node.left = insert(node.left, data);
         else
            node.right = insert(node.right, data);
      }
      return node;
   }
   private boolean search(BSTNode r, int val) {
      boolean found = false;
      while ((r != null) && !found) {
         int rval = r.data;
         if(val < rval)
            r = r.left;
         else if (val > rval)
            r = r.right;
         else {
            found = true;
            break;
         }
         found = search(r, val);
      }
      return found;
   }
   void printTree(BSTNode node, String prefix) {
      if(node == null)
         return;
      printTree(node.left , " " + prefix);
      System.out.println(prefix + "--" + node.data);
      printTree(node.right , prefix + " ");
   }
   public static void main(String args[]) {
      Scanner sc = new Scanner(System.in);
      BST bst = new BST();
      root = bst.insert(root, 55);
      root = bst.insert(root, 20);
      root = bst.insert(root, 90);
      root = bst.insert(root, 80);
      root = bst.insert(root, 50);
      root = bst.insert(root, 35);
      root = bst.insert(root, 15);
      root = bst.insert(root, 65);
      bst.printTree(root, " ");
      System.out.println("Element found = " + bst.search(root, 80));
   }
}

Output

--15
  --20--35
   --50
 --55
    --65
   --80
  --90
Element found = true

class Node:
   def __init__(self, data):
      self.left = None
      self.right = None
      self.data = data

# Insert method to create nodes
   def insert(self, data):
      if self.data:
         if data < self.data:
            if self.left is None:
               self.left = Node(data)
            else:
               self.left.insert(data)
         elif data > self.data:
            if self.right is None:
               self.right = Node(data)
            else:
               self.right.insert(data)
         else:
            self.data = data
# search method to compare the value with nodes
   def search(self, key):
      if key < self.data:
         if self.left is None:
            return str(key)+" Not Found"
         return self.left.search(key)
      elif key > self.data:
         if self.right is None:
            return str(key)+" Not Found"
         return self.right.search(key)
      else:
         print(str(self.data) + ' is found')

root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print(root.search(17))
print(root.search(12))

Output

17 Not Found
12 is found
None

Insert Operation

Whenever an element is to be inserted, first locate its proper location. Start searching from the root node, then if the data is less than the key value, search for the empty location in the left subtree and insert the data. Otherwise, search for the empty location in the right subtree and insert the data.

Algorithm

1 – START
2 – If the tree is empty, insert the first element as the root node of the tree. The following elements are added as the leaf nodes.
3 – If an element is less than the root value, it is added into the left subtree as a leaf node.
4 – If an element is greater than the root value, it is added into the right subtree as a leaf node.
5 – The final leaf nodes of the tree point to NULL values as their child nodes.
6 – END

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;
   
   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;
         
         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;
            
            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
void printTree(struct node* Node){
   if(Node == NULL)
      return;
   printTree(Node->leftChild);
   printf(" --%d", Node->data);
   printTree(Node->rightChild);
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printTree(root);
   return 0;
}

Output

Insertion done
 --15 --20 --35 --50 --55 --65 --90

#include <iostream>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;
   
   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;
         
         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;
            
            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
void printTree(struct node* Node){
   if(Node == NULL)
      return;
   printTree(Node->leftChild);
   printf(" --%d", Node->data);
   printTree(Node->rightChild);
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printTree(root);
   return 0;
}

Output

Insertion done
 --15 --20 --35 --50 --55 --65 --90

import java.util.Scanner;
class BSTNode {
   BSTNode left, right;
   int data;
   public BSTNode(int n) {
      left = null;
      right = null;
      data = n;
   }
}
public class BST {
   static BSTNode root;
   public BST() {
      root = null;
   }
   private BSTNode insert(BSTNode node, int data) {
      if(node == null)
         node = new BSTNode(data);
      else {
         if(data <= node.data)
            node.left = insert(node.left, data);
         else
            node.right = insert(node.right, data);
      }
      return node;
   }
   void printTree(BSTNode node, String prefix) {
      if(node == null)
         return;
      printTree(node.left , " " + prefix);
      System.out.println(prefix + "--" + node.data);
      printTree(node.right , prefix + " ");
   }
   public static void main(String args[]) {
      Scanner sc = new Scanner(System.in);
      BST bst = new BST();
      root = bst.insert(root, 55);
      root = bst.insert(root, 20);
      root = bst.insert(root, 90);
      root = bst.insert(root, 80);
      root = bst.insert(root, 50);
      root = bst.insert(root, 35);
      root = bst.insert(root, 15);
      root = bst.insert(root, 65);
      bst.printTree(root, " ");
   }
}

Output

--15
  --20
--35
   --50
 --55
    --65
   --80
  --90

class Node:
   def __init__(self, data):
      self.left = None
      self.right = None
      self.data = data

# Insert method to create nodes
   def insert(self, data):
      if self.data:
         if data < self.data:
            if self.left is None:
               self.left = Node(data)
            else:
               self.left.insert(data)
         elif data > self.data:
            if self.right is None:
               self.right = Node(data)
            else:
               self.right.insert(data)
         else:
            self.data = data
root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print("Insertion Done")

Output

Insertion Done

Inorder Traversal

The inorder traversal operation in a Binary Search Tree visits all its nodes in the following order −

  • Firstly, we traverse the left child of the root node/current node, if any.

  • Next, traverse the current node.

  • Lastly, traverse the right child of the current node, if any.

Algorithm

1. START
2. Traverse the left subtree, recursively
3. Then, traverse the root node
4. Traverse the right subtree, recursively.
5. END

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Inorder Traversal
void inorder(struct node *root){
   if (root != NULL) {
      inorder(root->left);
      printf("%d -> ", root->key);
      inorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
      node->left = insert(node->left, key);
   else
      node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Inorder traversal: ");
   inorder(root);
}

Output

Inorder traversal: 15 -> 20 -> 35 -> 50 -> 55 -> 65 -> 90 -> 

#include <iostream>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Inorder Traversal
void inorder(struct node *root){
   if (root != NULL) {
     inorder(root->left);
     printf("%d -> ", root->key);
     inorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
     node->left = insert(node->left, key);
   else
     node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Inorder traversal: ");
   inorder(root);
}

Output

Inorder traversal: 15 -> 20 -> 35 -> 50 -> 55 -> 65 -> 90 ->

class Node {
   int data;
   Node leftChild;
   Node rightChild;
   public Node(int key) {
      data = key;
      leftChild = rightChild = null;
   }
}
public class TreeDataStructure {
   Node root = null;
   void inorder_traversal(Node node) {
      if(node != null) {
         inorder_traversal(node.leftChild);
         System.out.print(node.data + " ");
         inorder_traversal(node.rightChild);
      }
   }
   public static void main(String args[]) {
      TreeDataStructure tree = new TreeDataStructure();
      tree.root = new Node(27);
      tree.root.leftChild = new Node(12);
      tree.root.rightChild = new Node(30);
      tree.root.leftChild.leftChild = new Node(4);
      tree.root.leftChild.rightChild = new Node(17);
      tree.root.rightChild.leftChild = new Node(56);
      System.out.println("\nInorder traversal: ");
      tree.inorder_traversal(tree.root);
   }
}

Output

Inorder traversal:
4 12 17 27 56 30

class Node:
   def __init__(self, data):
      self.left = None
      self.right = None
      self.data = data

# Insert method to create nodes
   def insert(self, data):
      if self.data:
         if data < self.data:
            if self.left is None:
               self.left = Node(data)
            else:
               self.left.insert(data)
         elif data > self.data:
            if self.right is None:
               self.right = Node(data)
            else:
               self.right.insert(data)
         else:
            self.data = data

# Print the tree
   def Inorder(self):
      if self.left:
         self.left.Inorder()
         print(self.data)
      if self.right:
         self.right.Inorder()

root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print("Inorder Traversal of Binary Search Tree: ")
root.Inorder()

Output

Inorder Traversal of Binary Search Tree: 
12
34
54

Preorder Traversal

The preorder traversal operation in a Binary Search Tree visits all its nodes. However, the root node in it is first printed, followed by its left subtree and then its right subtree.

Algorithm

1. START
2. Traverse the root node first.
3. Then traverse the left subtree, recursively
4. Later, traverse the right subtree, recursively.
5. END

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Preorder Traversal
void preorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->key);
      preorder(root->left);
      preorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
      node->left = insert(node->left, key);
   else
      node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Preorder traversal: ");
   preorder(root);
}

Output

Preorder traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

#include <iostream>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Preorder Traversal
void preorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->key);
      preorder(root->left);
      preorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
      node->left = insert(node->left, key);
   else
      node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Preorder traversal: ");
   preorder(root);
}

Output

Preorder traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

class Node {
    int data;
    Node leftChild;
    Node rightChild;
    public Node(int key) {
        data = key;
        leftChild = rightChild = null;
    }
}
public class TreeDataStructure {
    Node root = null;
    void preorder_traversal(Node node) {
        if(node != null) {
            System.out.print(node.data + " ");
            preorder_traversal(node.leftChild);
            preorder_traversal(node.rightChild);
        }
    }
    public static void main(String args[]) {
        TreeDataStructure tree = new TreeDataStructure();
        tree.root = new Node(27);
        tree.root.leftChild = new Node(12);
        tree.root.rightChild = new Node(30);
        tree.root.leftChild.leftChild = new Node(4);
        tree.root.leftChild.rightChild = new Node(17);
        tree.root.rightChild.leftChild = new Node(56);
        System.out.println("\nPreorder traversal: ");
        tree.preorder_traversal(tree.root);
    }
}

Output

Preorder traversal: 
27 12 4 17 30 56 

class Node:
   def __init__(self, data):
      self.left = None
      self.right = None
      self.data = data

# Insert method to create nodes
   def insert(self, data):
      if self.data:
         if data < self.data:
            if self.left is None:
               self.left = Node(data)
            else:
               self.left.insert(data)
         elif data > self.data:
            if self.right is None:
               self.right = Node(data)
            else:
               self.right.insert(data)
         else:
            self.data = data

# Print the tree
   def Preorder(self):
      print(self.data)
      if self.left:
         self.left.Preorder()
      if self.right:
         self.right.Preorder()
root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print("Preorder Traversal of Binary Search Tree: ")
root.Preorder()

Output

Preorder Traversal of Binary Search Tree: 
54
34
12
5
23
46

Postorder Traversal

Like the other traversals, postorder traversal also visits all the nodes in a Binary Search Tree and displays them. However, the left subtree is printed first, followed by the right subtree and lastly, the root node.

Algorithm

1. START
2. Traverse the left subtree, recursively
3. Traverse the right subtree, recursively.
4. Then, traverse the root node
5. END

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Postorder Traversal
void postorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->key);
      postorder(root->left);
      postorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
      node->left = insert(node->left, key);
   else
      node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Postorder traversal: ");
   postorder(root);
}

Output

Postorder traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

#include <iostream>
struct node {
   int key;
   struct node *left, *right;
};
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->key = item;
   temp->left = temp->right = NULL;
   return temp;
}

// Postorder Traversal
void postorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->key);
      postorder(root->left);
      postorder(root->right);
   }
}

// Insertion operation
struct node *insert(struct node *node, int key){
   if (node == NULL) return newNode(key);
   if (key < node->key)
      node->left = insert(node->left, key);
   else
      node->right = insert(node->right, key);
   return node;
}
int main(){
   struct node *root = NULL;
   root = insert(root, 55);
   root = insert(root, 20);
   root = insert(root, 90);
   root = insert(root, 50);
   root = insert(root, 35);
   root = insert(root, 15);
   root = insert(root, 65);
   printf("Postorder traversal: ");
   postorder(root);
}

Output

Postorder traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

class Node {
    int data;
    Node leftChild;
    Node rightChild;
    public Node(int key) {
        data = key;
        leftChild = rightChild = null;
    }
}
public class TreeDataStructure {
    Node root = null;
    void postorder_traversal(Node node) {
        if(node != null) {
            postorder_traversal(node.leftChild);
            postorder_traversal(node.rightChild);
            System.out.print(node.data + " ");
        }
    }
    public static void main(String args[]) {
        TreeDataStructure tree = new TreeDataStructure();
        tree.root = new Node(27);
        tree.root.leftChild = new Node(12);
        tree.root.rightChild = new Node(30);
        tree.root.leftChild.leftChild = new Node(4);
        tree.root.leftChild.rightChild = new Node(17);
        tree.root.rightChild.leftChild = new Node(56);
        System.out.println("\nPostorder traversal: ");
        tree.postorder_traversal(tree.root);
    }
}

Output

Postorder traversal: 
4 17 12 56 30 27 

class Node:
   def __init__(self, data):
      self.left = None
      self.right = None
      self.data = data

# Insert method to create nodes
   def insert(self, data):
      if self.data:
         if data < self.data:
            if self.left is None:
               self.left = Node(data)
            else:
               self.left.insert(data)
         elif data > self.data:
            if self.right is None:
               self.right = Node(data)
            else:
               self.right.insert(data)
      else:
         self.data = data

# Print the tree
   def Postorder(self):
      if self.left:
         self.left.Postorder()
      if self.right:
         self.right.Postorder()
      print(self.data)

root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print("Postorder Traversal of Binary Search Tree: ")
root.Postorder()

Output

Postorder Traversal of Binary Search Tree: 
5
23
12
46
34
54

Example

Following are the implementations of this operation in various programming languages −

C C++ Java Python 
#include <stdio.h>
#include <stdlib.h>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;

   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;

         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;

            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
struct node* search(int data){
   struct node *current = root;
   printf("\n\nVisiting elements: ");
   while(current->data != data) {
      if(current != NULL) {
         printf("%d ",current->data);

         //go to left tree
         if(current->data > data) {
            current = current->leftChild;
         }//else go to right tree
         else {
            current = current->rightChild;
         }

         //not found
         if(current == NULL) {
            return NULL;
         }
      }
   }
   return current;
}

// Inorder Traversal
void inorder(struct node *root){
   if (root != NULL) {
      inorder(root->leftChild);
      printf("%d -> ", root->data);
      inorder(root->rightChild);
   }
}

// Preorder Traversal
void preorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->data);
      preorder(root->leftChild);
      preorder(root->rightChild);
   }
}

// Postorder Traversal
void postorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->data);
      postorder(root->leftChild);
      postorder(root->rightChild);
   }
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printf("\nPreorder Traversal: ");
   preorder(root);
   printf("\nInorder Traversal: ");
   inorder(root);
   printf("\nPostorder Traversal: ");
   postorder(root);
   struct node* k;
   k = search(35);
   if(k != NULL)
      printf("\nElement %d found", k->data);
   else
      printf("\nElement not found");
   return 0;
}

Output

Insertion done

Preorder Traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 
Inorder Traversal: 15 -> 20 -> 35 -> 50 -> 55 -> 65 -> 90 -> 
Postorder Traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

Visiting elements: 55 20 50 
Element 35 found

#include <iostream>
struct node {
   int data;
   struct node *leftChild, *rightChild;
};
struct node *root = NULL;
struct node *newNode(int item){
   struct node *temp = (struct node *)malloc(sizeof(struct node));
   temp->data = item;
   temp->leftChild = temp->rightChild = NULL;
   return temp;
}
void insert(int data){
   struct node *tempNode = (struct node*) malloc(sizeof(struct node));
   struct node *current;
   struct node *parent;
   tempNode->data = data;
   tempNode->leftChild = NULL;
   tempNode->rightChild = NULL;
   
   //if tree is empty
   if(root == NULL) {
      root = tempNode;
   } else {
      current = root;
      parent = NULL;
      while(1) {
         parent = current;

         //go to left of the tree
         if(data < parent->data) {
            current = current->leftChild;

            //insert to the left
            if(current == NULL) {
               parent->leftChild = tempNode;
               return;
            }
         }//go to right of the tree
         else {
            current = current->rightChild;
            
            //insert to the right
            if(current == NULL) {
               parent->rightChild = tempNode;
               return;
            }
         }
      }
   }
}
struct node* search(int data){
   struct node *current = root;
   printf("\n\nVisiting elements: ");
   while(current->data != data) {
      if(current != NULL) {
         printf("%d ",current->data);
         
         //go to left tree
         if(current->data > data) {
            current = current->leftChild;
         }//else go to right tree
         else {
            current = current->rightChild;
         }
         
         //not found
         if(current == NULL) {
            return NULL;
         }
      }
   }
   return current;
}

// Inorder Traversal
void inorder(struct node *root){
   if (root != NULL) {
      inorder(root->leftChild);
      printf("%d -> ", root->data);
      inorder(root->rightChild);
   }
}

// Preorder Traversal
void preorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->data);
      preorder(root->leftChild);
      preorder(root->rightChild);
   }
}

// Postorder Traversal
void postorder(struct node *root){
   if (root != NULL) {
      printf("%d -> ", root->data);
      postorder(root->leftChild);
      postorder(root->rightChild);
   }
}
int main(){
   insert(55);
   insert(20);
   insert(90);
   insert(50);
   insert(35);
   insert(15);
   insert(65);
   printf("Insertion done\n");
   printf("\nPreorder Traversal: ");
   preorder(root);
   printf("\nInorder Traversal: ");
   inorder(root);
   printf("\nPostorder Traversal: ");
   postorder(root);
   struct node* k;
   k = search(35);
   if(k != NULL)
      printf("\nElement %d found", k->data);
   else
      printf("\nElement not found");
   return 0;
}

Output

Insertion done

Preorder Traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 
Inorder Traversal: 15 -> 20 -> 35 -> 50 -> 55 -> 65 -> 90 -> 
Postorder Traversal: 55 -> 20 -> 15 -> 50 -> 35 -> 90 -> 65 -> 

Visiting elements: 55 20 50 
Element 35 found

import java.util.Scanner;
class BSTNode {
   BSTNode left, right;
   int data;
   public BSTNode(int n) {
      left = null;
      right = null;
      data = n;
   }
}
public class BST {
   static BSTNode root;
   public BST() {
      root = null;
   }
   public boolean isEmpty() {
      return root == null;
   }
   private BSTNode insert(BSTNode node, int data) {
      if(node == null)
         node = new BSTNode(data);
      else {
         if(data <= node.data)
            node.left = insert(node.left, data);
         else
            node.right = insert(node.right, data);
      }
      return node;
   }
   public void delete(int k) {
      if(isEmpty ())
         System.out.println("TREE EMPTY");
      else if(search (k) == false)
         System.out.println("SORRY " + k + " IS NOT PRESENT");
      else {
         root=delete(root,k);
         System.out.println(k + " DELETED FROM THE TREE");
      }
   }
   public BSTNode delete(BSTNode root, int k) {
      BSTNode p, p2, n;
      if(root.data == k) {
         BSTNode lt, rt;
         lt = root.left;
         rt = root.right;
         if(lt == null && rt == null) {
            return null;
         } else if(lt == null) {
            p = rt;
            return p;
         } else if(rt == null) {
            p = lt;
            return p;
         } else {
            p2 = rt;
            p = rt;
            while(p.left != null)
               p = p.left;
            p.left = lt;
            return p2;
         }
      }
      if (k < root.data) {
         n = delete(root.left, k);
         root.left = n;
      } else {
         n = delete(root.right, k);
         root.right = n;
      }
      return root;
   }
   public boolean search(int val) {
      return search(root, val);
   }
   private boolean search(BSTNode r, int val) {
      boolean found = false;
      while ((r != null) && !found) {
         int rval = r.data;
         if(val < rval)
            r = r.left;
         else if (val > rval)
            r = r.right;
         else {
            found = true;
            break;
         }
         found = search(r, val);
      }
      return found;
   }
   void printTree(BSTNode node, String prefix) {
      if(node == null)
         return;
      printTree(node.left , " " + prefix);
      System.out.println(prefix + "--" + node.data);
      printTree(node.right , prefix + " ");
   }
   public static void main(String args[]) {
      Scanner sc = new Scanner(System.in);
      BST bst = new BST();
      root = bst.insert(root, 55);
      root = bst.insert(root, 20);
      root = bst.insert(root, 90);
      root = bst.insert(root, 80);
      root = bst.insert(root, 50);
      root = bst.insert(root, 35);
      root = bst.insert(root, 15);
      root = bst.insert(root, 65);
      bst.printTree(root, " ");
      bst.delete(55);
      System.out.println("Element found = " + bst.search(80));
      System.out.println("Is Tree Empty? " + bst.isEmpty());
   }
}

Output

--15
  --20--35
   --50
 --55
    --65
   --80
  --90
55 DELETED FROM THE TREE
Element found = true
Is Tree Empty? false

class Node:
   def __init__(self, data):
     self.left = None
     self.right = None
     self.data = data

# Insert method to create nodes
   def insert(self, data):
     if self.data:
       if data < self.data:
         if self.left is None:
            self.left = Node(data)
         else:
            self.left.insert(data)
       elif data > self.data:
         if self.right is None:
            self.right = Node(data)
         else:
            self.right.insert(data)
       else:
         self.data = data

# search method to compare the value with nodes
   def search(self, key):
     if key < self.data:
       if self.left is None:
         return str(key)+" Not Found"
       return self.left.search(key)
     elif key > self.data:
       if self.right is None:
         return str(key)+" Not Found"
       return self.right.search(key)
     else:
       print(str(self.data) + ' is found')

# Print the tree
   def Inorder(self):
     if self.left:
       self.left.Inorder()
     print(self.data)
     if self.right:
       self.right.Inorder()

# Print the tree
   def Preorder(self):
     print(self.data)
     if self.left:
       self.left.Preorder()
     if self.right:
       self.right.Preorder()

# Print the tree
   def Postorder(self):
     if self.left:
       self.left.Postorder()
     if self.right:
       self.right.Postorder()
     print(self.data)

root = Node(54)
root.insert(34)
root.insert(46)
root.insert(12)
root.insert(23)
root.insert(5)
print("Preorder Traversal of Binary Search Tree: ")
root.Preorder()
print("Inorder Traversal of Binary Search Tree: ")
root.Inorder()
print("Postorder Traversal of Binary Search Tree: ")
root.Postorder()
print(root.search(17))
print(root.search(12))

Output

Preorder Traversal of Binary Search Tree: 
54
34
12
5
23
46
Inorder Traversal of Binary Search Tree: 
5
12
23
34
46
54
Postorder Traversal of Binary Search Tree: 
5
23
12
46
34
54
17 Not Found
12 is found
None
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