Added Detect a Cycle in a graph

Author: MidhaShrey

CPP/graph_tree/Detect a cycle in a graph.cpp

@@ -0,0 +1,76 @@
+#include<iostream>
+#include<list>
+#include<cstring>
+using namespace std;
+
+class Graph{
+int V;
+list<int> *adj;
+
+bool isCyclicHelper(int s,bool *visited,bool *RStack){
+        if(visited[s]==false){
+                visited[s] = true;
+                rStack[s] = true;
+
+        for(auto it=adj[s].begin();it!=adj[s].end();it++){
+                if(!visited[*it]&&isCyclicHelper(*it,visited,rStack)){
+                    return true;
+                }
+                else if(rStack[*it]){
+                    return true;
+                }
+            }
+
+        }
+RStack[s]=false;
+return false;
+}
+
+
+public:
+   Graph(int V){
+        this->V = V;
+        adj = new list<int>[V];
+    }
+
+    void addEdge(int u,int v){
+        adj[u].push_back(v);
+
+    }
+
+    bool isCyclic(){
+        bool *visited = new bool[V];
+        bool *rStack = new bool[V];
+
+        memset(visited,false,sizeof(false));
+        memset(rStack,false,sizeof(false));
+
+        //This works if there is a single DFS forest. Otherwise use a for loop.
+        if(isCyclicHelper(0,visited,rStack)){
+            return true;
+        }
+        else{
+            return false;
+        }
+    }
+
+};
+
+int main(){
+
+ 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;
+
+return 0;
+}