From 1f1724e314ec9e854dce6b0ce54451dfc512d7c9 Mon Sep 17 00:00:00 2001
From: Sahil Tripathi <55251741+sahil2128@users.noreply.github.com>
Date: Wed, 28 Jul 2021 22:45:01 +0530
Subject: [PATCH] Create Detect Cycle in a Directed Graph.cpp

---
 Detect Cycle in a Directed Graph.cpp | 91 ++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 Detect Cycle in a Directed Graph.cpp

diff --git a/Detect Cycle in a Directed Graph.cpp b/Detect Cycle in a Directed Graph.cpp
new file mode 100644
index 0000000..8cb5c0a
--- /dev/null
+++ b/Detect Cycle in a Directed Graph.cpp	
@@ -0,0 +1,91 @@
+// A C++ Program to detect cycle in a graph
+#include<bits/stdc++.h>
+
+using namespace std;
+
+class Graph
+{
+	int V; // No. of vertices
+	list<int> *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<int>[V];
+}
+
+void Graph::addEdge(int v, int w)
+{
+	adj[v].push_back(w); // Add w to v’s list.
+}
+
+// This function is a variation of DFSUtil() in https://www.geeksforgeeks.org/archives/18212
+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 https://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;
+}