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Count of all cycles without any inner cycle in a given Graph
  • Last Updated : 16 Dec, 2020

Given an undirected graph consisting of N vertices numbered [0, N-1] and E edges, the task is to count the number of cycles such that any subset of vertices of a cycle does not form another cycle.
Examples: 
 

Input: N = 2, E = 2, edges = [{0, 1}, {1, 0}] 
Output:
Explanation: 
Only one cycle exists between the two vertices. 
 

Input: N = 6, E = 9, edges = [{0, 1}, {1, 2}, {0, 2}, {3, 0}, {3, 2}, {4, 1}, {4, 2}, {5, 1}, {5, 0}] 
Output:
Explanation: 
The possible cycles are shown in the diagram below: 
 

Example 2 Image



Cycles such as 5 -> 0 -> 2 -> 1 -> 5 are not considered as it comprises of inner cycles {5 -> 0 -> 1} and {0 -> 1 -> 2} 
 

Approach: 
Since V vertices require V edges to form 1 cycle, these the number of required cycles can be expressed using the formula: 
 

(Edges - Vertices) + 1

Illustration: 
 

N = 6, E = 9, edges = [{0, 1}, {1, 2}, {0, 2}, {3, 0}, {3, 2}, {4, 1}, {4, 2}, {5, 1}, {5, 0}] 
Number of Cycles = 9 – 6 + 1 = 4 
The 4 cycles in the graph are: 
{5, 0, 1}, {0, 1, 2}, {3, 0, 2} and {1, 2, 4} 
 

This formula also covers the case when a single vertex may have a self-loop. 
Below is the implementation of the above approach: 
 

C++




// C++ implementation for the
// above approach.
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to return the
// count of required cycles
int numberOfCycles(int N, int E,
                   int edges[][2])
{
    vector<int> graph[N];
    for (int i = 0; i < E; i++) {
        graph[edges[i][0]]
            .push_back(edges[i][1]);
        graph[edges[i][1]]
            .push_back(edges[i][0]);
    }
 
    // Return the number of cycles
    return (E - N) + 1;
}
 
// Driver Code
int main()
{
    int N = 6;
    int E = 9;
    int edges[][2] = { { 0, 1 },
                       { 1, 2 },
                       { 2, 0 },
                       { 5, 1 },
                       { 5, 0 },
                       { 3, 0 },
                       { 3, 2 },
                       { 4, 2 },
                       { 4, 1 } };
    int k = numberOfCycles(N, E,
                           edges);
 
    cout << k << endl;
    return 0;
}

Java




// Java implementation for the
// above approach.
import java.util.*;
 
class GFG{
 
// Function to return the
// count of required cycles
static int numberOfCycles(int N, int E,
                          int edges[][])
{
    @SuppressWarnings("unchecked")
    Vector<Integer> []graph = new Vector[N];
    for(int i = 0; i < N; i++)
        graph[i] = new Vector<Integer>();
         
    for(int i = 0; i < E; i++)
    {
        graph[edges[i][0]].add(edges[i][1]);
        graph[edges[i][1]].add(edges[i][0]);
    }
 
    // Return the number of cycles
    return (E - N) + 1;
}
 
// Driver Code
public static void main(String[] args)
{
    int N = 6;
    int E = 9;
    int edges[][] = { { 0, 1 },
                      { 1, 2 },
                      { 2, 0 },
                      { 5, 1 },
                      { 5, 0 },
                      { 3, 0 },
                      { 3, 2 },
                      { 4, 2 },
                      { 4, 1 } };
                       
    int k = numberOfCycles(N, E, edges);
 
    System.out.print(k + "\n");
}
}
 
// This code is contributed by Amit Katiyar

Python3




# Python3 implementation for the
# above approach.
  
# Function to return the
# count of required cycles
def numberOfCycles(N, E, edges):
 
    graph=[[] for i in range(N)]
     
    for i in range(E):
     
        graph[edges[i][0]].append(edges[i][1]);
        graph[edges[i][1]].append(edges[i][0]);
  
    # Return the number of cycles
    return (E - N) + 1;
  
# Driver Code
if __name__=='__main__':
     
    N = 6;
    E = 9;
    edges = [ [ 0, 1 ],
                       [ 1, 2 ],
                       [ 2, 0 ],
                       [ 5, 1 ],
                       [ 5, 0 ],
                       [ 3, 0 ],
                       [ 3, 2 ],
                       [ 4, 2 ],
                       [ 4, 1 ] ];
     
    k = numberOfCycles(N, E,edges);
    print(k)
     
    # This code is contributed by rutvik_56

C#




// C# implementation for the
// above approach.
using System;
using System.Collections.Generic;
class GFG{
 
// Function to return the
// count of required cycles
static int numberOfCycles(int N, int E,
                          int [,]edges)
{
 
    List<int> []graph = new List<int>[N];
    for(int i = 0; i < N; i++)
        graph[i] = new List<int>();
         
    for(int i = 0; i < E; i++)
    {
        graph[edges[i, 0]].Add(edges[i, 1]);
        graph[edges[i, 1]].Add(edges[i, 0]);
    }
 
    // Return the number of cycles
    return (E - N) + 1;
}
 
// Driver Code
public static void Main(String[] args)
{
    int N = 6;
    int E = 9;
    int [,]edges = { { 0, 1 }, { 1, 2 },
                     { 2, 0 }, { 5, 1 },
                     { 5, 0 }, { 3, 0 },
                     { 3, 2 }, { 4, 2 },
                     { 4, 1 } };
                       
    int k = numberOfCycles(N, E, edges);
 
    Console.Write(k + "\n");
}
}
 
// This code is contributed by Rohit_ranjan
Output: 
4

 

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

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