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Count the number of nodes at a given level in a tree using DFS
  • Difficulty Level : Basic
  • Last Updated : 01 Dec, 2020

Given an integer l and a tree represented as an undirected graph rooted at vertex 0. The task is to print the number of nodes present at level l.

Examples: 

Input: l = 2 
 

Output:



We have already discussed the BFS approach, in this post we will solve it using DFS.

Approach: The idea is to traverse the graph in a DFS manner. Take two variables, count and curr_level. Whenever the curr_level = l increment the value of the count.

Below is the implementation of the above approach: 

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
// Class to represent a graph
class Graph {
 
    // No. of vertices
    int V;
 
    // Pointer to an array containing
    // adjacency lists
    list<int>* adj;
 
    // A function used by NumOfNodes
    void DFS(vector<bool>& visited, int src, int& curr_level,
             int level, int& NumberOfNodes);
 
public:
    // Constructor
    Graph(int V);
 
    // Function to add an edge to graph
    void addEdge(int src, int des);
 
    // Returns the no. of nodes
    int NumOfNodes(int level);
};
 
Graph::Graph(int V)
{
    this->V = V;
    adj = new list<int>[V];
}
 
void Graph::addEdge(int src, int des)
{
    adj[src].push_back(des);
    adj[des].push_back(src);
}
 
// DFS function to keep track of
// number of nodes
void Graph::DFS(vector<bool>& visited, int src, int& curr_level,
                int level, int& NumberOfNodes)
{
    // Mark the current vertex as visited
    visited[src] = true;
 
    // If current level is equal
    // to the given level, increment
    // the no. of nodes
    if (level == curr_level) {
        NumberOfNodes++;
    }
    else if (level < curr_level)
        return;
    else {
        list<int>::iterator i;
 
        // Recur for the vertices
        // adjacent to the current vertex
        for (i = adj[src].begin(); i != adj[src].end(); i++) {
            if (!visited[*i]) {
                curr_level++;
                DFS(visited, *i, curr_level, level, NumberOfNodes);
            }
        }
    }
    curr_level--;
}
 
// Function to return the number of nodes
int Graph::NumOfNodes(int level)
{
    // To keep track of current level
    int curr_level = 0;
 
    // For keeping track of visited
    // nodes in DFS
    vector<bool> visited(V, false);
 
    // To store count of nodes at a
    // given level
    int NumberOfNodes = 0;
 
    DFS(visited, 0, curr_level, level, NumberOfNodes);
 
    return NumberOfNodes;
}
 
// Driver code
int main()
{
    int V = 8;
 
    Graph g(8);
    g.addEdge(0, 1);
    g.addEdge(0, 4);
    g.addEdge(0, 7);
    g.addEdge(4, 6);
    g.addEdge(4, 5);
    g.addEdge(4, 2);
    g.addEdge(7, 3);
 
    int level = 2;
 
    cout << g.NumOfNodes(level);
 
    return 0;
}

Python3




# Python3 implementation of the approach
  
# Class to represent a graph
class Graph:
     
    def __init__(self, V):
         
        # No. of vertices
        self.V = V
         
        # Pointer to an array containing
        # adjacency lists
        self.adj = [[] for i in range(self.V)]
         
    def addEdge(self, src, des):
         
        self.adj[src].append(des)
        self.adj[des].append(src)
         
    # DFS function to keep track of
    # number of nodes
    def DFS(self, visited, src, curr_level,
            level, NumberOfNodes):
 
        # Mark the current vertex as visited
        visited[src] = True
  
        # If current level is equal
        # to the given level, increment
        # the no. of nodes
        if (level == curr_level):
            NumberOfNodes += 1
     
        elif (level < curr_level):
            return
        else:
             
            # Recur for the vertices
            # adjacent to the current vertex
            for i in self.adj[src]:
         
                if (not visited[i]):
                    curr_level += 1
                    curr_level, NumberOfNodes = self.DFS(
                        visited, i, curr_level,
                        level, NumberOfNodes)
     
        curr_level -= 1
         
        return curr_level, NumberOfNodes
 
    # Function to return the number of nodes
    def NumOfNodes(self, level):
 
        # To keep track of current level
        curr_level = 0
  
        # For keeping track of visited
        # nodes in DFS
        visited = [False for i in range(self.V)]
     
        # To store count of nodes at a
        # given level
        NumberOfNodes = 0
  
        curr_level, NumberOfNodes = self.DFS(
            visited, 0, curr_level,
            level, NumberOfNodes)
  
        return NumberOfNodes
 
# Driver code
if __name__=='__main__':
 
    V = 8
  
    g = Graph(8)
    g.addEdge(0, 1)
    g.addEdge(0, 4)
    g.addEdge(0, 7)
    g.addEdge(4, 6)
    g.addEdge(4, 5)
    g.addEdge(4, 2)
    g.addEdge(7, 3)
  
    level = 2
  
    print(g.NumOfNodes(level))
  
# This code is contributed by pratham76
Output: 
4

 

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