Sudo Placement[1.4] | BST Traversal

Last Updated : 11 Jul, 2025

Given N elements to be inserted into Binary Search Tree. The task is to construct a binary search tree with only insert operation and finally print the elements in the postorder traversal. The BST is constructed according to the arrival order of elements.

Examples: 

Input: N elements =  {8, 5, 10, 3, 4, 9, 7}
Output: 4 3 7 5 9 10 8

For the above input, the BST is: 
               8
             /    \
          5         10
        /   \      /
       3     7    9
       \
        4
The post-order traversal of the above BST is 4 3 7 5 9 10 8

Approach: The approach to solve this problem is to construct the BST using insertion method in BST. Once all the nodes are inserted, print the postorder traversal of the tree.

Below is the implementation of the above approach:  

C++ 
// C++ program to insert nodes
// and print the postorder traversal
#include <bits/stdc++.h>
using namespace std;

// structure to store the BST
struct Node {
    int data;
    Node* left = NULL;
    Node* right = NULL;
};

// locates the memory space
Node* newNode(int key)
{
    Node* temp = new Node;
    temp->data = key;
    temp->left = NULL;
    temp->right = NULL;
    return temp;
}

// inserts node in the BST
Node* insertNode(Node* head, int key)
{
    // if first node
    if (head == NULL)
        head = newNode(key);
    else {
        // move to left
        if (key < head->data)
            head->left = insertNode(head->left, key);
        // move to right
        else
            head->right = insertNode(head->right, key);
    }
    return head;
}

// print the postorder traversal
void posOrder(Node* head)
{
    // leaf node is null
    if (head == NULL)
        return;

    // left
    posOrder(head->left);

    // right
    posOrder(head->right);

    // data
    cout << head->data << " ";
}

// Driver Code
int main()
{

    Node* root = NULL;
    root = insertNode(root, 8);
    root = insertNode(root, 5);
    root = insertNode(root, 10);
    root = insertNode(root, 3);
    root = insertNode(root, 4);
    root = insertNode(root, 9);
    root = insertNode(root, 7);

    // prints the postorder traversal of
    // the tree
    posOrder(root);
    cout << endl;
}
Java 
// Java program to insert nodes
// and print the postorder traversal
class GFG {

    // structure to store the BST
    static class Node {
        int data;
        Node left = null;
        Node right = null;
    };

    // locates the memory space
    static Node newNode(int key)
    {
        Node temp = new Node();
        temp.data = key;
        temp.left = null;
        temp.right = null;
        return temp;
    }

    // inserts node in the BST
    static Node insertNode(Node head, int key)
    {
        // if first node
        if (head == null)
            head = newNode(key);
        else {
            // move to left
            if (key < head.data)
                head.left = insertNode(head.left, key);
            // move to right
            else
                head.right = insertNode(head.right, key);
        }
        return head;
    }

    // print the postorder traversal
    static void posOrder(Node head)
    {
        // leaf node is null
        if (head == null)
            return;

        // left
        posOrder(head.left);

        // right
        posOrder(head.right);

        // data
        System.out.print(head.data + " ");
    }

    // Driver Code
    public static void main(String[] args)
    {
        Node root = new Node();
        root = insertNode(root, 8);
        root = insertNode(root, 5);
        root = insertNode(root, 10);
        root = insertNode(root, 3);
        root = insertNode(root, 4);
        root = insertNode(root, 9);
        root = insertNode(root, 7);

        // prints the postorder traversal of
        // the tree
        posOrder(root);
        System.out.println("");
    }
}

// This code has been contributed by 29AjayKumar
Python3 
# Python program to insert nodes
# and print the postorder traversal
import math

# structure to store the BST
class Node: 
    def __init__(self,data): 
        self.data = data 
        self.left = None
        self.right=None

# locates the memory space
def newNode(key):
    temp = Node(key)
    temp.data = key
    temp.left = None
    temp.right = None
    return temp

# inserts node in the BST
def insertNode(head, key):
    
    # if first node
    if (head == None):
        head = newNode(key)
    else :
        
        # move to left
        if (key < head.data):
            head.left = insertNode(head.left, key)
        
        # move to right
        else:
            head.right = insertNode(head.right, key)
    
    return head

# print the postorder traversal
def posOrder(head):
    
    # leaf node is None
    if (head == None):
        return

    # left
    posOrder(head.left)

    # right
    posOrder(head.right)

    # data
    print(head.data,end = " ")

# Driver Code
if __name__=='__main__': 
    root = None
    root = insertNode(root, 8)
    root = insertNode(root, 5)
    root = insertNode(root, 10)
    root = insertNode(root, 3)
    root = insertNode(root, 4)
    root = insertNode(root, 9)
    root = insertNode(root, 7)

    # prints the postorder traversal of
    # the tree
    posOrder(root)

# This code is contributed by Srathore
C# 
// C# program to insert nodes
// and print the postorder traversal
using System;

class GFG {

    // structure to store the BST
    public class Node {
        public int data;
        public Node left = null;
        public Node right = null;
    };

    // locates the memory space
    static Node newNode(int key)
    {
        Node temp = new Node();
        temp.data = key;
        temp.left = null;
        temp.right = null;
        return temp;
    }

    // inserts node in the BST
    static Node insertNode(Node head, int key)
    {
        // if first node
        if (head == null)
            head = newNode(key);
        else {
            // move to left
            if (key < head.data)
                head.left = insertNode(head.left, key);
            // move to right
            else
                head.right = insertNode(head.right, key);
        }
        return head;
    }

    // print the postorder traversal
    static void posOrder(Node head)
    {
        // leaf node is null
        if (head == null)
            return;

        // left
        posOrder(head.left);

        // right
        posOrder(head.right);

        // data
        Console.Write(head.data + " ");
    }

    // Driver Code
    public static void Main(String[] args)
    {
        Node root = new Node();
        root = insertNode(root, 8);
        root = insertNode(root, 5);
        root = insertNode(root, 10);
        root = insertNode(root, 3);
        root = insertNode(root, 4);
        root = insertNode(root, 9);
        root = insertNode(root, 7);

        // prints the postorder traversal of
        // the tree
        posOrder(root);
        Console.WriteLine("");
    }
}

// This code contributed by Rajput-Ji
JavaScript 
<script>

// Javascript program to insert nodes
// and print the postorder traversal

// Structure to store the BST
class Node 
{
    constructor()
    {
        this.data = 0;
        this.left = null;
        this.right = null;
    }
};

// Locates the memory space
function newNode(key)
{
    var temp = new Node();
    temp.data = key;
    temp.left = null;
    temp.right = null;
    return temp;
}

// Inserts node in the BST
function insertNode(head, key)
{
    
    // If first node
    if (head == null)
        head = newNode(key);
    else
    {
        
        // Move to left
        if (key < head.data)
            head.left = insertNode(head.left, key);
            
        // Move to right
        else
            head.right = insertNode(head.right, key);
    }
    return head;
}

// Print the postorder traversal
function posOrder(head)
{
    
    // Leaf node is null
    if (head == null)
        return;
        
    // Left
    posOrder(head.left);
    
    // Right
    posOrder(head.right);
    
    // Data
    document.write(head.data + " ");
}

// Driver Code
var root = null;
root = insertNode(root, 8);
root = insertNode(root, 5);
root = insertNode(root, 10);
root = insertNode(root, 3);
root = insertNode(root, 4);
root = insertNode(root, 9);
root = insertNode(root, 7);

// Prints the postorder traversal of
// the tree
posOrder(root);

document.write("<br>");

// This code is contributed by rutvik_56

</script>

Output
4 3 7 5 9 10 8
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