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

• Difficulty Level : Basic
• Last Updated : 19 Aug, 2022

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

Prerequisite: Tree Traversal

Examples:

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

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

The idea to solve this problem is to do preorder 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.

Implementation:

C++

 `// C++ program to find n-th node of ` `// Preorder Traversal of Binary Tree` `#include ` `using` `namespace` `std;`   `// Tree node` `struct` `Node {` `    ``int` `data;` `    ``Node *left, *right;` `};`   `// function to create 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 preorder` `// traversal of a given binary tree` `void` `NthPreordernode(``struct` `Node* root, ``int` `N)` `{` `    ``static` `int` `flag = 0;`   `    ``if` `(root == NULL)` `        ``return``;`   `    ``if` `(flag <= N) {` `        ``flag++;`   `        ``// prints the n-th node of preorder traversal` `        ``if` `(flag == N)` `            ``cout << root->data;`   `        ``// left recursion` `        ``NthPreordernode(root->left, N);`   `        ``// right recursion` `        ``NthPreordernode(root->right, N);` `    ``}` `}`   `// Driver code` `int` `main()` `{` `    ``// construction of binary tree` `    ``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);`   `    ``// nth node` `    ``int` `N = 6;`   `    ``// prints n-th  found` `    ``NthPreordernode(root, N);`   `    ``return` `0;` `}`

Java

 `// Java program to find n-th node of ` `// Preorder Traversal of Binary Tree` `class` `GFG` `{`   `// Tree node` `static` `class` `Node ` `{` `    ``int` `data;` `    ``Node left, right;` `};`   `// function to create new node` `static` `Node createNode(``int` `item)` `{` `    ``Node temp = ``new` `Node();` `    ``temp.data = item;` `    ``temp.left = ``null``;` `    ``temp.right = ``null``;`   `    ``return` `temp;` `}` `static` `int` `flag = ``0``;`   `// function to find the N-th node in the preorder` `// traversal of a given binary tree` `static` `void` `NthPreordernode(Node root, ``int` `N)` `{` `    `    `    ``if` `(root == ``null``)` `        ``return``;`   `    ``if` `(flag <= N)` `    ``{` `        ``flag++;`   `        ``// prints the n-th node of preorder traversal` `        ``if` `(flag == N)` `            ``System.out.print( root.data);`   `        ``// left recursion` `        ``NthPreordernode(root.left, N);`   `        ``// right recursion` `        ``NthPreordernode(root.right, N);` `    ``}` `}`   `// Driver code` `public` `static` `void` `main(String args[])` `{` `    ``// construction of binary tree` `    ``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``);`   `    ``// nth node` `    ``int` `N = ``6``;`   `    ``// prints n-th found` `    ``NthPreordernode(root, N);` `}` `}`   `// This code contributed by Arnab Kundu`

Python3

 `# Python program to find n-th node of ` `# Preorder Traversal of Binary Tree`   `class` `createNode(): `   `    ``def` `__init__(``self``, data): ` `        ``self``.data ``=` `data` `        ``self``.left ``=` `None` `        ``self``.right ``=` `None` `        `  `        `  `# function to find the N-th node in the preorder ` `# traversal of a given binary tree` `flag ``=` `[``0``]` `def` `NthPreordernode(root, N): ` `    ``if` `(root ``=``=` `None``):` `        ``return`   `    ``if` `(flag[``0``] <``=` `N): ` `        ``flag[``0``] ``+``=` `1`   `        ``# prints the n-th node of preorder traversal ` `        ``if` `(flag[``0``] ``=``=` `N):` `            ``print``(root.data)`   `        ``# left recursion ` `        ``NthPreordernode(root.left, N) `   `        ``# right recursion ` `        ``NthPreordernode(root.right, N)` `        `  `# Driver code ` `if` `__name__ ``=``=` `'__main__'``:` `    `  `    ``# construction of binary tree ` `    ``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``) `   `    ``# nth node ` `    ``N ``=` `6`   `    ``# prints n-th found  ` `    ``NthPreordernode(root, N)`   `# This code is contributed by SHUBHAMSINGH10`

C#

 `// C# program to find n-th node of ` `// Preorder Traversal of Binary Tree ` `using` `System;` `    `  `class` `GFG` `{`   `// Tree node` `public` `class` `Node ` `{` `    ``public` `int` `data;` `    ``public` `Node left, right;` `};`   `// function to create new node` `static` `Node createNode(``int` `item)` `{` `    ``Node temp = ``new` `Node();` `    ``temp.data = item;` `    ``temp.left = ``null``;` `    ``temp.right = ``null``;`   `    ``return` `temp;` `}` `static` `int` `flag = 0;`   `// function to find the N-th node in the preorder` `// traversal of a given binary tree` `static` `void` `NthPreordernode(Node root, ``int` `N)` `{` `    `    `    ``if` `(root == ``null``)` `        ``return``;`   `    ``if` `(flag <= N)` `    ``{` `        ``flag++;`   `        ``// prints the n-th node of preorder traversal` `        ``if` `(flag == N)` `            ``Console.Write( root.data);`   `        ``// left recursion` `        ``NthPreordernode(root.left, N);`   `        ``// right recursion` `        ``NthPreordernode(root.right, N);` `    ``}` `}`   `// Driver code` `public` `static` `void` `Main(String []args)` `{` `    ``// construction of binary tree` `    ``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);`   `    ``// nth node` `    ``int` `N = 6;`   `    ``// prints n-th found` `    ``NthPreordernode(root, N);` `}` `}`   `// This code is contributed by Rajput-Ji`

Javascript

 ``

Output

`24`

Complexity Analysis:

• Time Complexity: O(n), where n is the number of nodes in the given binary tree.
• Auxiliary Space: O(1)

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