# Cycle detection in directed graph.

How to detect cycle in directed graph Efficiently?

``````// A C++ Program to detect cycle in a graph
``````

#include
#include
#include <limits.h>

using namespace std;

class Graph
{
int V; // No. of vertices
bool isCyclicUtil(int v, bool visited[], bool *rs); // used by isCyclic()
public:
Graph(int V); // Constructor
void addEdge(int v, int w); // to add an edge to graph
bool isCyclic(); // returns true if there is a cycle in this graph
};

Graph::Graph(int V)
{
this->V = V;
}

{
}

bool Graph::isCyclicUtil(int v, bool visited[], bool *recStack)
{
if(visited[v] == false)
{
// Mark the current node as visited and part of recursion stack
visited[v] = true;
recStack[v] = true;

``````    // Recur for all the vertices adjacent to this vertex
list<int>::iterator i;
{
if ( !visited[*i] && isCyclicUtil(*i, visited, recStack) )
return true;
else if (recStack[*i])
return true;
}

}
recStack[v] = false;  // remove the vertex from recursion stack
return false;
``````

}

// Returns true if the graph contains a cycle, else false.
// This function is a variation of DFS() in http://www.geeksforgeeks.org/archives/18212
bool Graph::isCyclic()
{
// Mark all the vertices as not visited and not part of recursion
// stack
bool *visited = new bool[V];
bool *recStack = new bool[V];
for(int i = 0; i < V; i++)
{
visited[i] = false;
recStack[i] = false;
}

``````// Call the recursive helper function to detect cycle in different
// DFS trees
for(int i = 0; i < V; i++)
if (isCyclicUtil(i, visited, recStack))
return true;

return false;
``````

}

int main()
{
// Create a graph given in the above diagram
Graph g(4);

``````if(g.isCyclic())
cout << "Graph contains cycle";
else
cout << "Graph doesn't contain cycle";
return 0;
``````

}``

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Tnq @vpriyanshu40.

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