# Check if two nodes are cousins in a Binary Tree

• Difficulty Level : Medium
• Last Updated : 21 Nov, 2021

Given the binary Tree and the two nodes say ‘a’ and ‘b’, determine whether the two 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.

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The idea is to find level of one of the nodes. Using the found level, check if ‘a’ and ‘b’ are at this level. If ‘a’ and ‘b’ are at given level, then finally check if they are not children of same parent.
Following is the implementation of the above approach.

## C

 // C program to check if two Nodes in a binary tree are cousins#include #include  // A Binary Tree Nodestruct Node{    int data;    struct Node *left, *right;}; // A utility function to create a new Binary Tree Nodestruct Node *newNode(int item){    struct Node *temp =  (struct Node *)malloc(sizeof(struct Node));    temp->data = item;    temp->left = temp->right = NULL;    return temp;} // Recursive function to check if two Nodes are siblingsint isSibling(struct Node *root, struct Node *a, struct Node *b){    // Base case    if (root==NULL)  return 0;     return ((root->left==a && root->right==b)||            (root->left==b && root->right==a)||            isSibling(root->left, a, b)||            isSibling(root->right, a, b));} // Recursive function to find level of Node 'ptr' in a binary treeint level(struct Node *root, struct Node *ptr, int lev){    // base cases    if (root == NULL) return 0;    if (root == ptr)  return lev;     // Return level if Node is present in left subtree    int l = level(root->left, ptr, lev+1);    if (l != 0)  return l;     // Else search in right subtree    return level(root->right, ptr, lev+1);}  // Returns 1 if a and b are cousins, otherwise 0int isCousin(struct Node *root, struct Node *a, struct Node *b){    //1. The two Nodes should be on the same level in the binary tree.    //2. The two Nodes should not be siblings (means that they should    // not have the same parent Node).    if ((level(root,a,1) == level(root,b,1)) && !(isSibling(root,a,b)))        return 1;    else return 0;} // Driver Program to test above functionsint main(){    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 binary tree are cousinsclass Node{    int data;    Node left, right;     Node(int item)    {        data = item;        left = right = null;    }} class BinaryTree{    Node root;     // Recursive function to check if two Nodes are    // siblings    boolean isSibling(Node node, Node a, Node b)    {        // Base case        if (node == null)            return false;         return ((node.left == a && node.right == b) ||                (node.left == b && node.right == a) ||                isSibling(node.left, a, b) ||                isSibling(node.right, a, b));    }     // Recursive function to find level of Node 'ptr' in    // a binary tree    int level(Node node, Node ptr, int lev)    {        // base cases        if (node == null)            return 0;         if (node == ptr)            return lev;         // Return level if Node is present in left subtree        int l = level(node.left, ptr, lev + 1);        if (l != 0)            return l;         // Else search in right subtree        return level(node.right, ptr, lev + 1);    }     // Returns 1 if a and b are cousins, otherwise 0    boolean isCousin(Node node, Node a, Node b)    {        // 1. The two Nodes should be on the same level        //       in the binary tree.        // 2. The two Nodes should not be siblings (means        //    that they should not have the same parent        //    Node).        return ((level(node, a, 1) == level(node, b, 1)) &&                (!isSibling(node, a, b)));    }     //Driver program to test above functions    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.left;        Node2 = tree.root.right.right;        if (tree.isCousin(tree.root, Node1, Node2))            System.out.println("Yes");        else            System.out.println("No");    }} // This code has been contributed by Mayank Jaiswal

## Python

 # Python program to check if two nodes in a binary# tree are cousins # A Binary Tree Nodeclass Node:         # Constructor to create a new Binary Tree    def __init__(self, data):        self.data = data        self.left = None        self.right = None def isSibling(root, a , b):     # Base Case    if root is None:        return 0     return ((root.left == a and root.right ==b) or            (root.left == b and root.right == a)or            isSibling(root.left, a, b) or            isSibling(root.right, a, b)) # Recursive function to find level of Node 'ptr' in# a binary treedef level(root, ptr, lev):         # Base Case    if root is None :        return 0    if root == ptr:        return lev     # Return level if Node is present in left subtree    l = level(root.left, ptr, lev+1)    if l != 0:        return l     # Else search in right subtree    return level(root.right, ptr, lev+1)  # Returns 1 if a and b are cousins, otherwise 0def isCousin(root,a, b):         # 1. The two nodes should be on the same level in    # the binary tree    # The two nodes should not be siblings(means that    # they should not have the same parent node     if ((level(root,a,1) == level(root, b, 1)) and            not (isSibling(root, a, b))):        return 1    else:        return 0  # Driver program to test above functionroot = 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.rightnode2 = root.right.right print "Yes" if isCousin(root, node1, node2) == 1 else "No" # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

## C#

 // C# program to check if two binary// tree are cousinsusing System; public class Node{    public int data;    public Node left, right;     public Node(int item)    {        data = item;        left = right = null;    }} class GFG{public Node root; // Recursive function to check if// two Nodes are siblingspublic virtual bool isSibling(Node node,                              Node a, Node b){    // Base case    if (node == null)    {        return false;    }     return ((node.left == a && node.right == b) ||            (node.left == b && node.right == a) ||                     isSibling(node.left, a, b) ||                     isSibling(node.right, a, b));} // Recursive function to find level// of Node 'ptr' in a binary treepublic virtual int level(Node node, Node ptr, int lev){    // base cases    if (node == null)    {        return 0;    }     if (node == ptr)    {        return lev;    }     // Return level if Node is present    // in left subtree    int l = level(node.left, ptr, lev + 1);    if (l != 0)    {        return l;    }     // Else search in right subtree    return level(node.right, ptr, lev + 1);} // Returns 1 if a and b are cousins,// otherwise 0public virtual bool isCousin(Node node,                             Node a, Node b){    // 1. The two Nodes should be on the    //      same level in the binary tree.    // 2. The two Nodes should not be siblings    // (means that they should not have the    //  same parent Node).    return ((level(node, a, 1) == level(node, b, 1)) &&                            (!isSibling(node, a, b)));} // Driver Codepublic static void Main(string[] args){    GFG tree = new GFG();    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.left;    Node2 = tree.root.right.right;    if (tree.isCousin(tree.root, Node1, Node2))    {        Console.WriteLine("Yes");    }    else    {        Console.WriteLine("No");    }}} // This code is contributed by Shrikant13

## Javascript



Output:

Yes

Time Complexity of the above solution is O(n) as it does at most three traversals of binary tree.
Check if two nodes are cousins in a Binary Tree | Set-2

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