detect cycle in an undirected graph c++
// A C++ Program to detect
// cycle in an undirected graph
#include<iostream>
#include <list>
#include <limits.h>
using namespace std;
// Class for an undirected graph
class Graph
{
// No. of vertices
int V;
// Pointer to an array
// containing adjacency lists
list<int> *adj;
bool isCyclicUtil(int v, bool visited[],
int parent);
public:
// Constructor
Graph(int V);
// To add an edge to graph
void addEdge(int v, int w);
// Returns true if there is a cycle
bool isCyclic();
};
Graph::Graph(int V)
{
this->V = V;
adj = new list<int>[V];
}
void Graph::addEdge(int v, int w)
{
// Add w to v’s list.
adj[v].push_back(w);
// Add v to w’s list.
adj[w].push_back(v);
}
// A recursive function that
// uses visited[] and parent to detect
// cycle in subgraph reachable
// from vertex v.
bool Graph::isCyclicUtil(int v,
bool visited[], int parent)
{
// Mark the current node as visited
visited[v] = true;
// Recur for all the vertices
// adjacent to this vertex
list<int>::iterator i;
for (i = adj[v].begin(); i !=
adj[v].end(); ++i)
{
// If an adjacent vertex is not visited,
//then recur for that adjacent
if (!visited[*i])
{
if (isCyclicUtil(*i, visited, v))
return true;
}
// If an adjacent vertex is visited and
// is not parent of current vertex,
// then there exists a cycle in the graph.
else if (*i != parent)
return true;
}
return false;
}
// Returns true if the graph contains
// a cycle, else false.
bool Graph::isCyclic()
{
// Mark all the vertices as not
// visited and not part of recursion
// stack
bool *visited = new bool[V];
for (int i = 0; i < V; i++)
visited[i] = false;
// Call the recursive helper
// function to detect cycle in different
// DFS trees
for (int u = 0; u < V; u++)
{
// Don't recur for u if
// it is already visited
if (!visited[u])
if (isCyclicUtil(u, visited, -1))
return true;
}
return false;
}
// Driver program to test above functions
int main()
{
Graph g1(5);
g1.addEdge(1, 0);
g1.addEdge(0, 2);
g1.addEdge(2, 1);
g1.addEdge(0, 3);
g1.addEdge(3, 4);
g1.isCyclic()?
cout << "Graph contains cycle\n":
cout << "Graph doesn't contain cycle\n";
Graph g2(3);
g2.addEdge(0, 1);
g2.addEdge(1, 2);
g2.isCyclic()?
cout << "Graph contains cycle\n":
cout << "Graph doesn't contain cycle\n";
return 0;
}