Cycle detection in directed graph.

How to detect cycle in directed graph Efficiently?

maybe this can help you This is taken from this link

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

#include
#include
#include <limits.h>

using namespace std;

class Graph
{
int V; // No. of vertices
list *adj; // Pointer to an array containing adjacency lists
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;
adj = new list[V];
}

void Graph::addEdge(int v, int w)
{
adj[v].push_back(w); // Add w to vā€™s list.
}

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;
    for(i = adj[v].begin(); i != adj[v].end(); ++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);
g.addEdge(0, 1);
g.addEdge(0, 2);
g.addEdge(1, 2);
g.addEdge(2, 0);
g.addEdge(2, 3);
g.addEdge(3, 3);

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

}``

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Please share the link of any questions on this very topic on codechef. It will be of great help.

Tnq @vpriyanshu40.

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