Find n-th node in Postorder traversal of a Binary Tree

Given a Binary tree and a number N, write a program to find the N-th node in the Postorder traversal of the given Binary tree.

Prerequisite: Tree Traversal

Examples:

Input : N = 4
              11
            /   \
           21    31
         /   \
        41     51
Output : 31
Explanation: Postorder Traversal of given Binary Tree is 41 51 21 31 11, 
so 4th node will be 31.

Input : N = 5
             25
           /    \
          20    30
        /    \ /   \
      18    22 24   32
Output : 32

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

The idea to solve this problem is to do postorder traversal of the given binary tree and keep track of the count of nodes visited while traversing the tree and print the current node when the count becomes equal to N.

Below is the implementation of the above approach:

C++

// C++ program to find n-th node of 
// Postorder Traversal of Binary Tree 
#include <bits/stdc++.h> 
using namespace std; 
  
// node of tree 
struct Node { 
    int data; 
    Node *left, *right; 
}; 
  
// function to create a new node 
struct Node* createNode(int item) 
{ 
    Node* temp = new Node; 
    temp->data = item; 
    temp->left = NULL; 
    temp->right = NULL; 
  
    return temp; 
} 
  
// function to find the N-th node in the postorder 
// traversal of a given binary tree 
void NthPostordernode(struct Node* root, int N) 
{ 
    static int flag = 0; 
  
    if (root == NULL) 
        return; 
  
    if (flag <= N) { 
  
        // left recursion 
        NthPostordernode(root->left, N); 
  
        // right recursion 
        NthPostordernode(root->right, N); 
  
        flag++; 
  
        // prints the n-th node of preorder traversal 
        if (flag == N) 
            cout << root->data; 
    } 
} 
  
// driver code 
int main() 
{ 
    struct Node* root = createNode(25); 
    root->left = createNode(20); 
    root->right = createNode(30); 
    root->left->left = createNode(18); 
    root->left->right = createNode(22); 
    root->right->left = createNode(24); 
    root->right->right = createNode(32); 
  
    int N = 6; 
  
    // prints n-th node found 
    NthPostordernode(root, N); 
  
    return 0; 
} 

Java

// Java program to find n-th node of  
// Postorder Traversal of Binary Tree  
public class NthNodePostOrder { 
  
    static int flag = 0;  
  
    // function to find the N-th node in the postorder  
    // traversal of a given binary tree  
    public static void NthPostordernode(Node root, int N)  
    {  
    
        if (root == null)  
            return;  
    
        if (flag <= N)  
        {    
            // left recursion  
            NthPostordernode(root.left, N);  
            // right recursion  
            NthPostordernode(root.right, N);  
            flag++;  
            // prints the n-th node of preorder traversal  
            if (flag == N)  
                System.out.print(root.data); 
        }  
    }  
  
  
    public static void main(String args[]) { 
        Node root = new Node(25);  
        root.left = new Node(20);  
        root.right = new Node(30);  
        root.left.left = new Node(18);  
        root.left.right = new Node(22);  
        root.right.left = new Node(24);  
        root.right.right = new Node(32);  
    
        int N = 6;  
    
        // prints n-th node found  
        NthPostordernode(root, N); 
    } 
} 
  
/* A binary tree node structure */
class Node  
{  
    int data;  
    Node left, right;  
    Node(int data) 
    { 
        this.data=data; 
    } 
}; 
// This code is contributed by Gaurav Tiwari 

Python3

"""Python3 program to find n-th node of  
Postorder Traversal of Binary Tree"""
  
# A Binary Tree Node  
# Utility function to create a new tree node  
class createNode:  
  
    # Constructor to create a newNode  
    def __init__(self, data):  
        self.data= data  
        self.left = None
        self.right = None
  
# function to find the N-th node  
# in the postorder traversal of 
# a given binary tree 
flag = [0] 
def NthPostordernode(root, N):  
  
    if (root == None): 
        return
  
    if (flag[0] <= N[0]): 
          
        # left recursion  
        NthPostordernode(root.left, N)  
  
        # right recursion  
        NthPostordernode(root.right, N)  
  
        flag[0] += 1
  
        # prints the n-th node of 
        # preorder traversal  
        if (flag[0] == N[0]): 
            print(root.data) 
                          
# Driver Code 
if __name__ == '__main__': 
    root = createNode(25)  
    root.left = createNode(20)  
    root.right = createNode(30)  
    root.left.left = createNode(18)  
    root.left.right = createNode(22)  
    root.right.left = createNode(24)  
    root.right.right = createNode(32)  
  
    N = [6]  
  
    # prints n-th node found  
    NthPostordernode(root, N) 
  
# This code is contributed by  
# SHUBHAMSINGH10 

C#

// C# program to find n-th node of  
// Postorder Traversal of Binary Tree  
using System; 
  
public class NthNodePostOrder  
{ 
  
    /* A binary tree node structure */
    public class Node  
    {  
        public int data;  
        public Node left, right;  
        public Node(int data) 
        { 
            this.data=data; 
        } 
    } 
    static int flag = 0;  
  
    // function to find the N-th node in the postorder  
    // traversal of a given binary tree  
    static void NthPostordernode(Node root, int N)  
    {  
        if (root == null)  
            return;  
      
        if (flag <= N)  
        {  
            // left recursion  
            NthPostordernode(root.left, N);  
              
            // right recursion  
            NthPostordernode(root.right, N);  
            flag++;  
              
            // prints the n-th node of preorder traversal  
            if (flag == N)  
                Console.Write(root.data); 
        }  
    }  
  
    // Driver code 
    public static void Main(String []args) 
    { 
        Node root = new Node(25);  
        root.left = new Node(20);  
        root.right = new Node(30);  
        root.left.left = new Node(18);  
        root.left.right = new Node(22);  
        root.right.left = new Node(24);  
        root.right.right = new Node(32);  
      
        int N = 6;  
      
        // prints n-th node found  
        NthPostordernode(root, N); 
    } 
} 
  
// This code is contributed by Arnab Kundu 