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Check if two nodes are cousins in a Binary Tree | Set-2
• Difficulty Level : Medium
• Last Updated : 13 Jun, 2019

Given a binary tree and the two nodes say ‘a’ and ‘b’, determine whether two given nodes are cousins of each other or not.

Two nodes are cousins of each other if they are at same level and have different parents.

Example:

```     6
/   \
3     5
/ \   / \
7   8 1   3
Say two node be 7 and 1, result is TRUE.
Say two nodes are 3 and 5, result is FALSE.
Say two nodes are 7 and 5, result is FALSE.```

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

A solution in Set-1 that finds whether given nodes are cousins or not by performing three traversals of binary tree has been discussed. The problem can be solved by performing level order traversal. The idea is to use a queue to perform level order traversal, in which each queue element is a pair of node and parent of that node. For each node visited in level order traversal, check if that node is either first given node or second given node. If any node is found store parent of that node. While performing level order traversal, one level is traversed at a time. If both nodes are found in given level, then their parent values are compared to check if they are siblings or not. If one node is found in given level and another is not found, then given nodes are not cousins.

Below is the implementation of above approach:

## C++

 `// CPP program to check if two Nodes in``// a binary tree are cousins``// using level-order traversals``#include ``using` `namespace` `std;`` ` `// A Binary Tree Node``struct` `Node {``    ``int` `data;``    ``struct` `Node *left, *right;``};`` ` `// A utility function to create a new``// Binary Tree Node``struct` `Node* newNode(``int` `item)``{``    ``struct` `Node* temp = (``struct` `Node*)``malloc``(``sizeof``(``struct` `Node));``    ``temp->data = item;``    ``temp->left = temp->right = NULL;``    ``return` `temp;``}`` ` `// Returns true if a and b are cousins,``// otherwise false.``bool` `isCousin(Node* root, Node* a, Node* b)``{``    ``if` `(root == NULL)``        ``return` `false``;`` ` `    ``// To store parent of node a.``    ``Node* parA = NULL;`` ` `    ``// To store parent of node b.``    ``Node* parB = NULL;`` ` `    ``// queue to perform level order``    ``// traversal. Each element of``    ``// queue is a pair of node and``    ``// its parent.``    ``queue > q;`` ` `    ``// Dummy node to act like parent``    ``// of root node.``    ``Node* tmp = newNode(-1);`` ` `    ``// To store front element of queue.``    ``pair ele;`` ` `    ``// Push root to queue.``    ``q.push(make_pair(root, tmp));``    ``int` `levSize;`` ` `    ``while` `(!q.empty()) {`` ` `        ``// find number of elements in``        ``// current level.``        ``levSize = q.size();``        ``while` `(levSize) {`` ` `            ``ele = q.front();``            ``q.pop();`` ` `            ``// check if current node is node a``            ``// or node b or not.``            ``if` `(ele.first->data == a->data) {``                ``parA = ele.second;``            ``}`` ` `            ``if` `(ele.first->data == b->data) {``                ``parB = ele.second;``            ``}`` ` `            ``// push children of current node``            ``// to queue.``            ``if` `(ele.first->left) {``                ``q.push(make_pair(ele.first->left, ele.first));``            ``}`` ` `            ``if` `(ele.first->right) {``                ``q.push(make_pair(ele.first->right, ele.first));``            ``}`` ` `            ``levSize--;`` ` `            ``// If both nodes are found in``            ``// current level then no need``            ``// to traverse current level further.``            ``if` `(parA && parB)``                ``break``;``        ``}`` ` `        ``// Check if both nodes are siblings``        ``// or not.``        ``if` `(parA && parB) {``            ``return` `parA != parB;``        ``}`` ` `        ``// If one node is found in current level``        ``// and another is not found, then``        ``// both nodes are not cousins.``        ``if` `((parA && !parB) || (parB && !parA)) {``            ``return` `false``;``        ``}``    ``}`` ` `    ``return` `false``;``}``// Driver Code``int` `main()``{``    ``/*``            ``1 ``           ``/  \ ``          ``2    3``         ``/ \  / \``        ``4   5 6  7``             ``\ \``             ``15 8``    ``*/`` ` `    ``struct` `Node* root = newNode(1);``    ``root->left = newNode(2);``    ``root->right = newNode(3);``    ``root->left->left = newNode(4);``    ``root->left->right = newNode(5);``    ``root->left->right->right = newNode(15);``    ``root->right->left = newNode(6);``    ``root->right->right = newNode(7);``    ``root->right->left->right = newNode(8);`` ` `    ``struct` `Node *Node1, *Node2;``    ``Node1 = root->left->left;``    ``Node2 = root->right->right;`` ` `    ``isCousin(root, Node1, Node2) ? ``puts``(``"Yes"``) : ``puts``(``"No"``);`` ` `    ``return` `0;``}`

## Java

 `// Java program to check if two Nodes in``// a binary tree are cousins``// using level-order traversals``import` `java.util.*;``import` `javafx.util.Pair;`` ` `// User defined node class``class` `Node``{``    ``int` `data;``    ``Node left, right;`` ` `    ``// Constructor to create a new tree node ``    ``Node(``int` `item)``    ``{``        ``data = item;``        ``left = right = ``null``;``    ``}``}`` ` `class` `BinaryTree``{``    ``Node root;`` ` `    ``// Returns true if a and b are cousins, ``    ``// otherwise false. ``    ``boolean` `isCousin(Node node, Node a, Node b)``    ``{``        ``if``(node == ``null``)``            ``return` `false``;``         ` `        ``// To store parent of node a.``        ``Node parA = ``null``;`` ` `        ``// To store parent of node b.``        ``Node parB = ``null``;`` ` `        ``// queue to perform level order ``        ``// traversal. Each element of ``        ``// queue is a pair of node and ``        ``// its parent.``        ``Queue> q = ``new` `LinkedList<> ();`` ` `        ``// Dummy node to act like parent ``        ``// of root node. ``        ``Node tmp = ``new` `Node(-``1``);`` ` `        ``// To store front element of queue. ``        ``Pair ele;`` ` `        ``// Push root to queue.``        ``q.add(``new` `Pair (node, tmp));`` ` `        ``int` `levelSize;`` ` `        ``while``(!q.isEmpty())``        ``{`` ` `            ``// find number of elements in ``            ``// current level. ``            ``levelSize = q.size();``            ``while``(levelSize != ``0``)``            ``{``                ``ele = q.peek();``                ``q.remove();`` ` `                ``// check if current node is node a ``                ``// or node b or not. ``                ``if``(ele.getKey().data == a.data)``                    ``parA = ele.getValue();`` ` `                ``if``(ele.getKey().data == b.data)``                    ``parB = ele.getValue();`` ` `                ``// push children of current node ``                ``// to queue. ``                ``if``(ele.getKey().left != ``null``)``                    ``q.add(``new` `Pair(ele.getKey().left, ele.getKey()));`` ` `                ``if``(ele.getKey().right != ``null``)``                    ``q.add(``new` `Pair(ele.getKey().right, ele.getKey()));`` ` `                ``levelSize--;`` ` `                ``// If both nodes are found in ``                ``// current level then no need ``                ``// to traverse current level further. ``                ``if``(parA != ``null` `&& parB != ``null``)``                    ``break``;``            ``}`` ` `            ``// Check if both nodes are siblings ``            ``// or not.``            ``if``(parA != ``null` `&& parB != ``null``)``                ``return` `parA != parB;`` ` `            ``// If one node is found in current level ``            ``// and another is not found, then ``            ``// both nodes are not cousins. ``            ``if` `((parA!=``null` `&& parB==``null``) || (parB!=``null` `&& parA==``null``))``                ``return` `false``;``        ``}`` ` `        ``return` `false``;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``BinaryTree tree = ``new` `BinaryTree();``        ``tree.root = ``new` `Node(``1``);``        ``tree.root.left = ``new` `Node(``2``);``        ``tree.root.right = ``new` `Node(``3``);``        ``tree.root.left.left = ``new` `Node(``4``);``        ``tree.root.left.right = ``new` `Node(``5``);``        ``tree.root.left.right.right = ``new` `Node(``15``);``        ``tree.root.right.left = ``new` `Node(``6``);``        ``tree.root.right.right = ``new` `Node(``7``);``        ``tree.root.right.left.right = ``new` `Node(``8``);`` ` `        ``Node Node1, Node2;``        ``Node1 = tree.root.left.right.right;``        ``Node2 = tree.root.right.left.right;``        ``if` `(tree.isCousin(tree.root, Node1, Node2))``            ``System.out.println(``"Yes"``);``        ``else``            ``System.out.println(``"No"``);``    ``}``}`` ` `// This code is contributed by shubham96301`

## Python3

 `# Python3 program to check if two ``# Nodes in a binary tree are cousins ``# using level-order traversals `` ` `# A Binary Tree Node ``class` `Node:  ``     ` `    ``def` `__init__(``self``, item):``        ``self``.data ``=` `item``        ``self``.left ``=` `None``        ``self``.right ``=` `None`` ` `# Returns True if a and b ``# are cousins, otherwise False. ``def` `isCousin(root, a, b): ``  ` `    ``if` `root ``=``=` `None``:``        ``return` `False` ` ` `    ``# To store parent of node a. ``    ``parA ``=` `None` ` ` `    ``# To store parent of node b. ``    ``parB ``=` `None` ` ` `    ``# queue to perform level order ``    ``# traversal. Each element of queue ``    ``# is a pair of node and its parent. ``    ``q ``=` `[] `` ` `    ``# Dummy node to act like ``    ``# parent of root node. ``    ``tmp ``=` `Node(``-``1``) `` ` `    ``# Push root to queue. ``    ``q.append((root, tmp)) ``     ` `    ``while` `len``(q) > ``0``:  `` ` `        ``# find number of elements in ``        ``# current level. ``        ``levSize ``=` `len``(q) ``        ``while` `levSize:  `` ` `            ``ele ``=` `q.pop(``0``) `` ` `            ``# check if current node is ``            ``# node a or node b or not. ``            ``if` `ele[``0``].data ``=``=` `a.data:  ``                ``parA ``=` `ele[``1``] ``              ` `            ``if` `ele[``0``].data ``=``=` `b.data:  ``                ``parB ``=` `ele[``1``] `` ` `            ``# push children of ``            ``# current node to queue. ``            ``if` `ele[``0``].left:  ``                ``q.append((ele[``0``].left, ele[``0``])) ``              ` `            ``if` `ele[``0``].right:  ``                ``q.append((ele[``0``].right, ele[``0``])) ``            ``levSize ``-``=` `1`` ` `            ``# If both nodes are found in ``            ``# current level then no need ``            ``# to traverse current level further. ``            ``if` `parA ``and` `parB: ``                ``break` ` ` `        ``# Check if both nodes ``        ``# are siblings or not. ``        ``if` `parA ``and` `parB:  ``            ``return` `parA !``=` `parB `` ` `        ``# If one node is found in current level ``        ``# and another is not found, then ``        ``# both nodes are not cousins. ``        ``if` `(parA ``and` `not` `parB) ``or` `(parB ``and` `not` `parA): ``            ``return` `False` `          ` `    ``return` `False` `  ` `# Driver Code ``if` `__name__ ``=``=` `'__main__'``:`` ` `    ``root ``=` `Node(``1``) ``    ``root.left ``=` `Node(``2``) ``    ``root.right ``=` `Node(``3``) ``    ``root.left.left ``=` `Node(``4``) ``    ``root.left.right ``=` `Node(``5``) ``    ``root.left.right.right ``=` `Node(``15``) ``    ``root.right.left ``=` `Node(``6``) ``    ``root.right.right ``=` `Node(``7``) ``    ``root.right.left.right ``=` `Node(``8``) `` ` `    ``Node1 ``=` `root.left.left ``    ``Node2 ``=` `root.right.right `` ` `    ``if` `isCousin(root, Node1, Node2):``        ``print``(``'Yes'``)``    ``else``:``        ``print``(``'No'``)``         ` `# This code is contributed by Rituraj Jain`

## C#

 `// C# program to check if two Nodes in``// a binary tree are cousins``// using level-order traversals``using` `System;``using` `System.Collections.Generic; `` ` `// User defined node class``public` `class` `Node``{``    ``public` `int` `data;``    ``public` `Node left, right;`` ` `    ``// Constructor to create a new tree node ``    ``public` `Node(``int` `item)``    ``{``        ``data = item;``        ``left = right = ``null``;``    ``}``}``// User defined pair class``public` `class` `Pair``{`` ` `    ``public` `Node first, second;`` ` `    ``// Constructor to create a new tree node ``    ``public` `Pair(Node first, Node second)``    ``{``        ``this``.first = first;``        ``this``.second = second;``    ``}``}`` ` `class` `BinaryTree``{``    ``Node root;`` ` `    ``// Returns true if a and b are cousins, ``    ``// otherwise false. ``    ``Boolean isCousin(Node node, Node a, Node b)``    ``{``        ``if``(node == ``null``)``            ``return` `false``;``         ` `        ``// To store parent of node a.``        ``Node parA = ``null``;`` ` `        ``// To store parent of node b.``        ``Node parB = ``null``;`` ` `        ``// queue to perform level order ``        ``// traversal. Each element of ``        ``// queue is a pair of node and ``        ``// its parent.``        ``Queue q = ``new` `Queue ();`` ` `        ``// Dummy node to act like parent ``        ``// of root node. ``        ``Node tmp = ``new` `Node(-1);`` ` `        ``// To store front element of queue. ``        ``Pair ele;`` ` `        ``// Push root to queue.``        ``q.Enqueue(``new` `Pair (node, tmp));`` ` `        ``int` `levelSize;`` ` `        ``while``(q.Count>0)``        ``{`` ` `            ``// find number of elements in ``            ``// current level. ``            ``levelSize = q.Count;``            ``while``(levelSize != 0)``            ``{``                ``ele = q.Peek();``                ``q.Dequeue();`` ` `                ``// check if current node is node a ``                ``// or node b or not. ``                ``if``(ele.first.data == a.data)``                    ``parA = ele.second;`` ` `                ``if``(ele.first.data == b.data)``                    ``parB = ele.second;`` ` `                ``// push children of current node ``                ``// to queue. ``                ``if``(ele.first.left != ``null``)``                    ``q.Enqueue(``new` `Pair(ele.first.left, ele.first));`` ` `                ``if``(ele.first.right != ``null``)``                    ``q.Enqueue(``new` `Pair(ele.first.right, ele.first));`` ` `                ``levelSize--;`` ` `                ``// If both nodes are found in ``                ``// current level then no need ``                ``// to traverse current level further. ``                ``if``(parA != ``null` `&& parB != ``null``)``                    ``break``;``            ``}`` ` `            ``// Check if both nodes are siblings ``            ``// or not.``            ``if``(parA != ``null` `&& parB != ``null``)``                ``return` `parA != parB;`` ` `            ``// If one node is found in current level ``            ``// and another is not found, then ``            ``// both nodes are not cousins. ``            ``if` `((parA != ``null` `&& parB == ``null``) || (parB != ``null` `&& parA == ``null``))``                ``return` `false``;``        ``}`` ` `        ``return` `false``;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `Main(String []args)``    ``{``        ``BinaryTree tree = ``new` `BinaryTree();``        ``tree.root = ``new` `Node(1);``        ``tree.root.left = ``new` `Node(2);``        ``tree.root.right = ``new` `Node(3);``        ``tree.root.left.left = ``new` `Node(4);``        ``tree.root.left.right = ``new` `Node(5);``        ``tree.root.left.right.right = ``new` `Node(15);``        ``tree.root.right.left = ``new` `Node(6);``        ``tree.root.right.right = ``new` `Node(7);``        ``tree.root.right.left.right = ``new` `Node(8);`` ` `        ``Node Node1, Node2;``        ``Node1 = tree.root.left.right.right;``        ``Node2 = tree.root.right.left.right;``        ``if` `(tree.isCousin(tree.root, Node1, Node2))``            ``Console.WriteLine(``"Yes"``);``        ``else``            ``Console.WriteLine(``"No"``);``    ``}``}`` ` `// This code is contributed by Arnab Kundu`
Output:
```Yes
```

Time Complexity: O(n)
Auxiliary Space: O(n)

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