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Find the path from root to the given nodes of a tree for multiple queries
  • Difficulty Level : Hard
  • Last Updated : 29 Oct, 2020

Given a tree with N vertices numbered from 0 to N – 1 (0th node is the root node). Also, given q queries containing nodes in the tree. The task is to find the path from the root node to the given node for multiple queries.

Examples: 

Input: N = 6, q[] = {2, 4}
Tree:
                    0
                   / \
                  1   2
                  |
                  3
                 / \
                4   5
Output:
0 2
0 1 3 4
The path from root node to node 2 is 0 -> 2.
The path from root node to node 4 is 0 -> 1 -> 3 -> 4.

Approach: The path from any root vertex to any vertex ‘i’ is the path from the root vertex to its parent followed by the parent itself. This can be achieved by modifying the Breadth-First-Traversal of the tree. In the path list, for each unvisited vertex, add the copy of the path of its parent to its list and then add the parent to the list.

Below is the implementation of the above approach: 

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
const int sz = 1e5;
 
// Adjacency list representation
// of the tree
vector<int> tree[sz];
 
// Boolean array to mark all the
// vertices which are visited
bool vis[sz];
 
// Array of vector where ith index
// stores the path from the root
// node to the ith node
vector<int> path[sz];
 
// Utility 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
    // the queue and mark as visited
    qu.push({ node, -1 });
    vis[node] = true;
 
    while (!qu.empty()) {
        pair<int, int> p = qu.front();
 
        // Dequeue a vertex from queue
        qu.pop();
        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 });
 
                // Path from the root to this vertex is
                // the path from root to the parent
                // of this vertex followed by the
                // parent itself
                path[child] = path[p.first];
                path[child].push_back(p.first);
            }
        }
    }
}
 
// Utility Function to print the
// path from root to given node
void displayPath(int node)
{
    vector<int> ans = path[node];
    for (int k : ans) {
        cout << k << " ";
    }
    cout << node << '\n';
}
 
// Driver code
int main()
{
 
    // Number of vertices
    int n = 6;
 
    addEdge(0, 1);
    addEdge(0, 2);
    addEdge(1, 3);
    addEdge(3, 4);
    addEdge(3, 5);
 
    // Calling modified bfs function
    bfs(0);
 
    // Display paths from root vertex
    // to the given vertices
    displayPath(2);
    displayPath(4);
    displayPath(5);
 
    return 0;
}

Java




// Java implementation of the approach
import java.util.*;
 
@SuppressWarnings("unchecked")
class GFG {
 
    static class Pair<T, V> {
        T first;
        V second;
 
        Pair() {
        }
 
        Pair(T first, V second) {
            this.first = first;
            this.second = second;
        }
    }
 
    static int sz = (int) 1e5;
 
    // Adjacency list representation
    // of the tree
    static Vector<Integer>[] tree = new Vector[sz];
 
    // Boolean array to mark all the
    // vertices which are visited
    static boolean[] vis = new boolean[sz];
 
    // Array of vector where ith index
    // stores the path from the root
    // node to the ith node
    static Vector<Integer>[] path = new Vector[sz];
 
    // Utility 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<Integer, Integer>> qu = new LinkedList<>();
 
        // Push root node in the front of
        // the queue and mark as visited
        qu.add(new Pair<>(node, -1));
        vis[node] = true;
 
        while (!qu.isEmpty()) {
 
            // Dequeue a vertex from queue
            Pair<Integer, Integer> p = qu.poll();
 
            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));
 
                    // Path from the root to this vertex is
                    // the path from root to the parent
                    // of this vertex followed by the
                    // parent itself
                    path[child] = (Vector<Integer>) path[p.first].clone();
                    path[child].add(p.first);
                }
            }
        }
    }
 
    // Utility Function to print the
    // path from root to given node
    static void displayPath(int node) {
        for (int k : path[node]) {
            System.out.print(k + " ");
        }
        System.out.println(node);
    }
 
    // Driver Code
    public static void main(String[] args) {
        for (int i = 0; i < sz; i++) {
            tree[i] = new Vector<>();
            path[i] = new Vector<>();
            vis[i] = false;
        }
 
        // Number of vertices
        int n = 6;
 
        addEdge(0, 1);
        addEdge(0, 2);
        addEdge(1, 3);
        addEdge(3, 4);
        addEdge(3, 5);
 
        // Calling modified bfs function
        bfs(0);
 
        // Display paths from root vertex
        // to the given vertices
        displayPath(2);
        displayPath(4);
        displayPath(5);
    }
}
 
// This code is contributed by
// sanjeev2552

Python3




# Python3 implementation of the approach
from collections import deque as queue
 
sz = 7
 
# Adjacency list representation
# of the tree
tree = [[] for i in range(sz)]
 
# Boolean array to mark all the
# vertices which are visited
vis = [False] * sz
 
# Array of vector where ith index
# stores the path from the root
# node to the ith node
path = [[] for i in range(sz)]
 
# Utility 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 = queue()
 
    # Push root node in the front of
    # the queue and mark as visited
    qu.append([node, -1])
    vis[node] = True
 
    while (len(qu) > 0):
        p = qu.popleft()
         
        #print(p,p[0],p[1])
 
        # Dequeue a vertex from queue
        # qu.pop()
        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 (vis[child] == False):
                qu.append([child, p[0]])
 
                # Path from the root to this
                # vertex is the path from root
                # to the parent of this vertex
                # followed by the parent itself
                for u in path[p[0]]:
                    path[child].append(u)
 
                path[child].append(p[0])
                 
                #print(child,":",path[0])
 
# Utility Function to prthe
# path from root to given node
def displayPath(node):
     
    ans = path[node]
    for k in ans:
        print(k, end = " ")
         
    print(node)
 
# Driver code
if __name__ == '__main__':
 
    # Number of vertices
    n = 6
 
    addEdge(0, 1)
    addEdge(0, 2)
    addEdge(1, 3)
    addEdge(3, 4)
    addEdge(3, 5)
 
    # Calling modified bfs function
    bfs(0)
 
    # Display paths from root vertex
    # to the given vertices
    displayPath(2)
    displayPath(4)
    displayPath(5)
 
# This code is contributed by mohit kumar 29
Output: 
0 2
0 1 3 4
0 1 3 5


 

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