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Print cousins of a given node in Binary Tree | Single Traversal
  • Difficulty Level : Medium
  • Last Updated : 10 Dec, 2020

Given a binary tree and a node, print all cousins of given node. Note that siblings should not be printed.
Examples: 
 

Input : root of below tree 
             1
           /   \
          2     3
        /   \  /  \
       4    5  6   7
       and pointer to a node say 5.

Output : 6, 7

 

Note that it is the same problem as given at Print cousins of a given node in Binary Tree which consists of two traversals recursively. In this post, a single level traversal approach is discussed.
The idea is to go for level order traversal of the tree, as the cousins and siblings of a node can be found in its level order traversal. Run the traversal till the level containing the node is not found, and if found, print the given level.
How to print the cousin nodes instead of siblings and how to get the nodes of that level in the queue? During the level order, when for the parent node, if parent->left == Node_to_find, or parent->right == Node_to_find, then the children of this parent must not be pushed into the queue (as one is the node and other will be its sibling). Push the remaining nodes at the same level in the queue and then exit the loop. The current queue will have the nodes at the next level (the level of the node being searched, except the node and its sibling). Now, print the queue.
Following is the implementation of the above algorithm.
 

C++




// CPP program to print cousins of a node
#include <iostream>
#include <queue>
using namespace std;
 
// A Binary Tree Node
struct Node {
    int data;
    Node *left, *right;
};
 
// A utility function to create a new Binary
// Tree Node
Node* newNode(int item)
{
    Node* temp = new Node;
    temp->data = item;
    temp->left = temp->right = NULL;
    return temp;
}
 
// function to print cousins of the node
void printCousins(Node* root, Node* node_to_find)
{
    // if the given node is the root itself,
    // then no nodes would be printed
    if (root == node_to_find) {
        cout << "Cousin Nodes : None" << endl;
        return;
    }
 
    queue<Node*> q;
    bool found = false;
    int size_;
    Node* p;
    q.push(root);
 
    // the following loop runs until found is
    // not true, or q is not empty.
    // if found has become true => we have found
    // the level in which the node is present
    // and the present queue will contain all the
    // cousins of that node
    while (!q.empty() && !found) {
 
        size_ = q.size();
        while (size_) {
            p = q.front();
            q.pop();
 
            // if current node's left or right child
            // is the same as the node to find,
            // then make found = true, and don't push
            // any of them into the queue, as
            // we don't have to print the siblings
            if ((p->left == node_to_find ||
                p->right == node_to_find)) {
                found = true;
            }
            else {
                if (p->left)
                    q.push(p->left);
                if (p->right)
                    q.push(p->right);
            }
 
            size_--;
        }
    }
 
    // if found == true then the queue will contain the
    // cousins of the given node
    if (found) {
        cout << "Cousin Nodes : ";
        size_ = q.size();
 
        // size_ will be 0 when the node was at the
        // level just below the root node.
        if (size_ == 0)
            cout << "None";
        for (int i = 0; i < size_; i++) {
            p = q.front();
            q.pop();
            cout << p->data << " ";
        }
    }
    else {
        cout << "Node not found";
    }
    cout << endl;
    return;
}
 
// Driver Program to test above function
int main()
{
    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);
 
    Node* x = newNode(43);
 
    printCousins(root, x);
    printCousins(root, root);
    printCousins(root, root->right);
    printCousins(root, root->left);
    printCousins(root, root->left->right);
 
    return 0;
}

Java




// Java program to print
// cousins of a node
import java.io.*;
import java.util.*;
import java.lang.*;
 
 
// A Binary Tree Node
class Node
{
    int data;
    Node left, right;
    Node(int key)
    {
        data = key;
        left = right = null;
    }
}
 
class GFG
{
     
// function to print
// cousins of the node
static void printCousins(Node root,
                         Node node_to_find)
{
    // if the given node
    // is the root itself,
    // then no nodes would
    // be printed
    if (root == node_to_find)
    {
        System.out.print("Cousin Nodes :" +
                           " None" + "\n");
        return;
    }
 
    Queue<Node> q = new LinkedList<Node>();
    boolean found = false;
    int size_ = 0;
    Node p = null;
    q.add(root);
 
    // the following loop runs
    // until found is not true,
    // or q is not empty. if
    // found has become true => we
    // have found the level in
    // which the node is present
    // and the present queue will
    // contain all the cousins of
    // that node
    while (q.isEmpty() == false &&
                 found == false)
    {
 
        size_ = q.size();
        while (size_ -- > 0)
        {
            p = q.peek();
            q.remove();
 
            // if current node's left
            // or right child is the
            // same as the node to find,
            // then make found = true,
            // and don't push any of them
            // into the queue, as we don't
            // have to print the siblings
            if ((p.left == node_to_find ||
                 p.right == node_to_find))
            {
                found = true;
            }
            else
            {
                if (p.left != null)
                    q.add(p.left);
                if (p.right!= null)
                    q.add(p.right);
            }
 
        }
    }
 
    // if found == true then the
    // queue will contain the
    // cousins of the given node
    if (found == true)
    {
        System.out.print("Cousin Nodes : ");
        size_ = q.size();
 
        // size_ will be 0 when
        // the node was at the
        // level just below the
        // root node.
        if (size_ == 0)
            System.out.print("None");
         
        for (int i = 0; i < size_; i++)
        {
            p = q.peek();
            q.poll();
             
            System.out.print(p.data + " ");
        }
    }
    else
    {
        System.out.print("Node not found");
    }
     
    System.out.println("");
    return;
}
 
// Driver code
public static void main(String[] args)
{
    Node root = new Node(1);
    root.left = new Node(2);
    root.right = new Node(3);
    root.left.left = new Node(4);
    root.left.right = new Node(5);
    root.left.right.right = new Node(15);
    root.right.left = new Node(6);
    root.right.right = new Node(7);
    root.right.left.right = new Node(8);
 
    Node x = new Node(43);
 
    printCousins(root, x);
    printCousins(root, root);
    printCousins(root, root.right);
    printCousins(root, root.left);
    printCousins(root, root.left.right);
}
}

Python3




# Python3 program to print cousins of a node
  
# A Binary Tree Node
class Node:
     
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
     
# A utility function to create a new Binary
# Tree Node
def newNode(item):
 
    temp = Node(item)
    return temp
     
# function to print cousins of the node
def printCousins(root, node_to_find):
 
    # if the given node is the root itself,
    # then no nodes would be printed
    if (root == node_to_find):
        print("Cousin Nodes : None")
        return;
  
    q = []
    found = False;
    size_ = 0
    p = None
    q.append(root);
  
    # the following loop runs until found is
    # not true, or q is not empty.
    # if found has become true => we have found
    # the level in which the node is present
    # and the present queue will contain all the
    # cousins of that node
    while (len(q) != 0 and not found):
  
        size_ = len(q)
         
        while (size_ != 0):
             
            p = q[0]
            q.pop(0);
  
            # if current node's left or right child
            # is the same as the node to find,
            # then make found = true, and don't append
            # any of them into the queue, as
            # we don't have to print the siblings
            if ((p.left == node_to_find or p.right == node_to_find)):
                found = True;
            else:
                if (p.left):
                    q.append(p.left);
                if (p.right):
                    q.append(p.right);
  
            size_-=1
         
    # if found == true then the queue will contain the
    # cousins of the given node
    if (found):
        print("Cousin Nodes : ", end='')
        size_ = len(q)
  
        # size_ will be 0 when the node was at the
        # level just below the root node.
        if (size_ == 0):
            print("None", end='')
         
        for i in range(0, size_):
         
            p = q[0]
            q.pop(0);
            print(p.data, end=' ')
     
    else:
        print("Node not found", end='')
         
    print()
    return;
  
# Driver Program to test above function
if __name__=='__main__':
     
    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);
  
    x = newNode(43);
  
    printCousins(root, x);
    printCousins(root, root);
    printCousins(root, root.right);
    printCousins(root, root.left);
    printCousins(root, root.left.right);
  
# This code is contributed by rutvik_56

C#




// C# program to print
// cousins of a node
using System;
using System.Collections.Generic;
 
// A Binary Tree Node
public class Node
{
    public int data;
    public Node left, right;
    public Node(int key)
    {
        data = key;
        left = right = null;
    }
}
  
public class GFG
{
      
// function to print
// cousins of the node
static void printCousins(Node root,
                         Node node_to_find)
{
    // if the given node
    // is the root itself,
    // then no nodes would
    // be printed
    if (root == node_to_find)
    {
        Console.Write("Cousin Nodes :" +
                           " None" + "\n");
        return;
    }
  
    Queue<Node> q = new Queue<Node>();
    bool found = false;
    int size_ = 0;
    Node p = null;
    q.Enqueue(root);
  
    // the following loop runs
    // until found is not true,
    // or q is not empty. if
    // found has become true => we
    // have found the level in
    // which the node is present
    // and the present queue will
    // contain all the cousins of
    // that node
    while (q.Count!=0 &&
                 found == false)
    {
  
        size_ = q.Count;
        while (size_ -- > 0)
        {
            p = q.Peek();
            q.Dequeue();
  
            // if current node's left
            // or right child is the
            // same as the node to find,
            // then make found = true,
            // and don't push any of them
            // into the queue, as we don't
            // have to print the siblings
            if ((p.left == node_to_find ||
                 p.right == node_to_find))
            {
                found = true;
            }
            else
            {
                if (p.left != null)
                    q.Enqueue(p.left);
                if (p.right!= null)
                    q.Enqueue(p.right);
            }
  
        }
    }
  
    // if found == true then the
    // queue will contain the
    // cousins of the given node
    if (found == true)
    {
        Console.Write("Cousin Nodes : ");
        size_ = q.Count;
  
        // size_ will be 0 when
        // the node was at the
        // level just below the
        // root node.
        if (size_ == 0)
            Console.Write("None");
          
        for (int i = 0; i < size_; i++)
        {
            p = q.Peek();
            q.Dequeue();
              
            Console.Write(p.data + " ");
        }
    }
    else
    {
        Console.Write("Node not found");
    }
      
    Console.WriteLine("");
    return;
}
  
// Driver code
public static void Main(String[] args)
{
    Node root = new Node(1);
    root.left = new Node(2);
    root.right = new Node(3);
    root.left.left = new Node(4);
    root.left.right = new Node(5);
    root.left.right.right = new Node(15);
    root.right.left = new Node(6);
    root.right.right = new Node(7);
    root.right.left.right = new Node(8);
  
    Node x = new Node(43);
  
    printCousins(root, x);
    printCousins(root, root);
    printCousins(root, root.right);
    printCousins(root, root.left);
    printCousins(root, root.left.right);
}
}
 
// This code is contributed Rajput-Ji
Output: 
Node not found
Cousin Nodes : None
Cousin Nodes : None
Cousin Nodes : None
Cousin Nodes : 6 7

 

Time Complexity : This is a single level order traversal, hence time complexity = O(n), and Auxiliary space = O(n) (See this).
 

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