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Find parent of each node in a tree for multiple queries

  • Last Updated : 04 Aug, 2021

Given a tree with N vertices numbered from 0 to N – 1 and Q query containing nodes in the tree, the task is to find the parent node of the given node for multiple queries. Consider the 0th node as the root node and take the parent of the root node as the root itself.
Examples: 
 

Tree:
      0
     /  \
    1    2
    |   / \
    3  4   5

Input: N = 2
Output: 0
Explanation:
Parent of node 2 is node 0 i.e root node

Input: N = 3
Output: 1
Explanation:
Parent of node 3 is node 1

 

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Approach
By default, we assign the parent of the root node as the root itself. Then, we traverse the tree using Breadth First Traversal(BFS). When we mark the children of node s as visited, we also assign the parent node of these children as the node s. Finally, for different queries, the value of the parent[] of the node is printed.
Below is the implementation of above approach:
 

C++




// C++ implementation for
// the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
const int sz = 1e5;
 
// Adjacency list representation
// of the tree
vector<int> tree[sz + 1];
 
// Boolean array to mark all the
// vertices which are visited
bool vis[sz + 1];
 
// Array of vector where ith index
// stores the path from the root
// node to the ith node
int ans[sz + 1];
 
// Function to create an
// edge between two vertices
void addEdge(int a, int b)
{
 
    // Add a to b's list
    tree[a].push_back(b);
 
    // Add b to a's list
    tree[b].push_back(a);
}
 
// Modified Breadth-First Function
void bfs(int node)
{
 
    // Create a queue of {child, parent}
    queue<pair<int, int> > qu;
 
    // Push root node in the front of
    qu.push({ node, 0 });
 
    while (!qu.empty()) {
        pair<int, int> p = qu.front();
 
        // Dequeue a vertex from queue
        qu.pop();
        ans[p.first] = p.second;
        vis[p.first] = true;
 
        // Get all adjacent vertices of the dequeued
        // vertex s. If any adjacent has not
        // been visited then enqueue it
        for (int child : tree[p.first]) {
            if (!vis[child]) {
                qu.push({ child, p.first });
            }
        }
    }
}
 
// Driver code
int main()
{
 
    // Number of vertices
    int n = 6;
 
    addEdge(0, 1);
    addEdge(0, 2);
    addEdge(1, 3);
    addEdge(2, 4);
    addEdge(2, 5);
 
    // Calling modified bfs function
    bfs(0);
 
    int q[] = { 2, 3 };
 
    for (int i = 0; i < 2; i++) {
        cout << ans[q[i]] << '\n';
    }
 
    return 0;
}

Java




// Java implementation for
// the above approach
import java.util.*;
 
class GFG
{
static int sz = (int) 1e5;
 
// Adjacency list representation
// of the tree
static Vector<Integer> []tree = new Vector[sz + 1];
 
// Boolean array to mark all the
// vertices which are visited
static boolean []vis = new boolean[sz + 1];
 
// Array of vector where ith index
// stores the path from the root
// node to the ith node
static int []ans = new int[sz + 1];
static class pair
{
    int first, second;
    public pair(int first, int second)
    {
        this.first = first;
        this.second = second;
    }
}
 
// Function to create an
// edge between two vertices
static void addEdge(int a, int b)
{
 
    // Add a to b's list
    tree[a].add(b);
 
    // Add b to a's list
    tree[b].add(a);
}
 
// Modified Breadth-First Function
static void bfs(int node)
{
 
    // Create a queue of {child, parent}
    Queue<pair> qu = new LinkedList<>();
 
    // Push root node in the front of
    qu.add(new pair(node, 0 ));
 
    while (!qu.isEmpty())
    {
        pair p = qu.peek();
 
        // Dequeue a vertex from queue
        qu.remove();
        ans[p.first] = p.second;
        vis[p.first] = true;
 
        // Get all adjacent vertices of the dequeued
        // vertex s. If any adjacent has not
        // been visited then enqueue it
        for (int child : tree[p.first])
        {
            if (!vis[child])
            {
                qu.add(new pair(child, p.first ));
            }
        }
    }
}
 
// Driver code
public static void main(String[] args)
{
     
    // Number of vertices
    int n = 6;
    for (int i = 0; i < sz + 1; i++)
        tree[i] = new Vector<Integer>();
         
    addEdge(0, 1);
    addEdge(0, 2);
    addEdge(1, 3);
    addEdge(2, 4);
    addEdge(2, 5);
 
    // Calling modified bfs function
    bfs(0);
 
    int q[] = { 2, 3 };
 
    for (int i = 0; i < 2; i++)
    {
        System.out.println(ans[q[i]]);
    }
}
}
 
// This code is contributed by 29AjayKumar

Python




# Python implementation for
# the above approach
 
sz = 10**5
 
# Adjacency list representation
# of the tree
tree = [[] for _ in range(sz + 1)]
 
# Boolean array to mark all the
# vertices which are visited
vis = [0] * (sz + 1)
 
# Array of vector where ith index
# stores the path from the root
# node to the ith node
ans = [0] * (sz + 1)
 
# Function to create an
# edge between two vertices
def addEdge(a, b):
     
    # Add a to b's list
    tree[a].append(b)
     
    # Add b to a's list
    tree[b].append(a)
 
# Modified Breadth-First Function
def bfs(node):
     
    # Create a queue of child, parent
    qu = []
     
    # Push root node in the front of
    qu.append([node, 0])
     
    while (len(qu)):
        p = qu[0]
         
        # Dequeue a vertex from queue
        qu.pop(0)
        ans[p[0]] = p[1]
        vis[p[0]] = True
         
        # Get all adjacent vertices of the dequeued
        # vertex s. If any adjacent has not
        # been visited then enqueue it
        for child in tree[p[0]]:
            if (not vis[child]):
                qu.append([child, p[0]])
                 
# Driver code
 
# Number of vertices
n = 6
 
addEdge(0, 1)
addEdge(0, 2)
addEdge(1, 3)
addEdge(2, 4)
addEdge(2, 5)
 
# Calling modified bfs function
bfs(0)
 
q = [2, 3]
 
for i in range(2):
    print(ans[q[i]])
 
# This code is contributed by SHUBHAMSINGH10

C#




// C# implementation of the approach
using System;
using System.Collections.Generic;
 
class GFG
{
static int sz = (int) 1e5;
 
// Adjacency list representation
// of the tree
static List<int> []tree = new List<int>[sz + 1];
 
// Boolean array to mark all the
// vertices which are visited
static Boolean []vis = new Boolean[sz + 1];
 
// Array of vector where ith index
// stores the path from the root
// node to the ith node
static int []ans = new int[sz + 1];
public class pair
{
    public int first, second;
    public pair(int first, int second)
    {
        this.first = first;
        this.second = second;
    }
}
 
// Function to create an
// edge between two vertices
static void addEdge(int a, int b)
{
 
    // Add a to b's list
    tree[a].Add(b);
 
    // Add b to a's list
    tree[b].Add(a);
}
 
// Modified Breadth-First Function
static void bfs(int node)
{
 
    // Create a queue of {child, parent}
    Queue<pair> qu = new Queue<pair>();
 
    // Push root node in the front of
    qu.Enqueue(new pair(node, 0 ));
 
    while (qu.Count != 0)
    {
        pair p = qu.Peek();
 
        // Dequeue a vertex from queue
        qu.Dequeue();
        ans[p.first] = p.second;
        vis[p.first] = true;
 
        // Get all adjacent vertices of the dequeued
        // vertex s. If any adjacent has not
        // been visited then enqueue it
        foreach (int child in tree[p.first])
        {
            if (!vis[child])
            {
                qu.Enqueue(new pair(child, p.first ));
            }
        }
    }
}
 
// Driver code
public static void Main(String[] args)
{
     
    // Number of vertices
    for (int i = 0; i < sz + 1; i++)
        tree[i] = new List<int>();
         
    addEdge(0, 1);
    addEdge(0, 2);
    addEdge(1, 3);
    addEdge(2, 4);
    addEdge(2, 5);
 
    // Calling modified bfs function
    bfs(0);
 
    int []q = { 2, 3 };
 
    for (int i = 0; i < 2; i++)
    {
        Console.WriteLine(ans[q[i]]);
    }
}
}
 
// This code is contributed by 29AjayKumar

Javascript




<script>
     
    // JavaScript implementation for the above approach
     
    let sz = 1e5;
   
    // Adjacency list representation
    // of the tree
    let tree = new Array(sz + 1);
 
    // Boolean array to mark all the
    // vertices which are visited
    let vis = new Array(sz + 1);
    vis.fill(false);
 
    // Array of vector where ith index
    // stores the path from the root
    // node to the ith node
    let ans = new Array(sz + 1);
    ans.fill(0);
 
    // Function to create an
    // edge between two vertices
    function addEdge(a, b)
    {
 
        // Add a to b's list
        tree[a].push(b);
 
        // Add b to a's list
        tree[b].push(a);
    }
 
    // Modified Breadth-First Function
    function bfs(node)
    {
 
        // Create a queue of {child, parent}
        let qu = [];
 
        // Push root node in the front of
        qu.push([node, 0 ]);
 
        while (qu.length > 0)
        {
            let p = qu[0];
 
            // Dequeue a vertex from queue
            qu.shift();
            ans[p[0]] = p[1];
            vis[p[0]] = true;
 
            // Get all adjacent vertices of the dequeued
            // vertex s. If any adjacent has not
            // been visited then enqueue it
            for (let child = 0; child < tree[p[0]].length; child++)
            {
                if (!vis[tree[p[0]][child]])
                {
                    qu.push([tree[p[0]][child], p[0]]);
                }
            }
        }
    }
     
    // Number of vertices
    let n = 6;
    for (let i = 0; i < sz + 1; i++)
        tree[i] = [];
           
    addEdge(0, 1);
    addEdge(0, 2);
    addEdge(1, 3);
    addEdge(2, 4);
    addEdge(2, 5);
   
    // Calling modified bfs function
    bfs(0);
   
    let q = [ 2, 3 ];
   
    for (let i = 0; i < 2; i++)
    {
        document.write(ans[q[i]] + "</br>");
    }
 
</script>
Output: 
0
1

 

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




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