From ecd7fb35bbed526fd60d7938ba3dbbc0f6ee3f52 Mon Sep 17 00:00:00 2001
From: sumeshpandit <32913357+sumeshpandit@users.noreply.github.com>
Date: Sat, 17 Oct 2020 14:05:21 +0530
Subject: [PATCH] Create tarjan's algorithm.cpp
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Tarjan’s Algorithm to find Strongly Connected Components
---
 C++/tarjan's algorithm.cpp | 166 +++++++++++++++++++++++++++++++++++++
 1 file changed, 166 insertions(+)
 create mode 100644 C++/tarjan's algorithm.cpp

diff --git a/C++/tarjan's algorithm.cpp b/C++/tarjan's algorithm.cpp
new file mode 100644
index 0000000..95bca67
--- /dev/null
+++ b/C++/tarjan's algorithm.cpp	
@@ -0,0 +1,166 @@
+// A C++ program to find strongly connected components in a given 
+// directed graph using Tarjan's algorithm (single DFS) 
+#include<iostream> 
+#include <list> 
+#include <stack> 
+#define NIL -1 
+using namespace std; 
+
+// A class that represents an directed graph 
+class Graph 
+{ 
+	int V; // No. of vertices 
+	list<int> *adj; // A dynamic array of adjacency lists 
+
+	// A Recursive DFS based function used by SCC() 
+	void SCCUtil(int u, int disc[], int low[], 
+				stack<int> *st, bool stackMember[]); 
+public: 
+	Graph(int V); // Constructor 
+	void addEdge(int v, int w); // function to add an edge to graph 
+	void SCC(); // prints strongly connected components 
+}; 
+
+Graph::Graph(int V) 
+{ 
+	this->V = V; 
+	adj = new list<int>[V]; 
+} 
+
+void Graph::addEdge(int v, int w) 
+{ 
+	adj[v].push_back(w); 
+} 
+
+void Graph::SCCUtil(int u, int disc[], int low[], stack<int> *st, 
+					bool stackMember[]) 
+{ 
+	static int time = 0; 
+
+	// Initialize discovery time and low value 
+	disc[u] = low[u] = ++time; 
+	st->push(u); 
+	stackMember[u] = true; 
+
+	// Go through all vertices adjacent to this 
+	list<int>::iterator i; 
+	for (i = adj[u].begin(); i != adj[u].end(); ++i) 
+	{ 
+		int v = *i; // v is current adjacent of 'u' 
+
+		// If v is not visited yet, then recur for it 
+		if (disc[v] == -1) 
+		{ 
+			SCCUtil(v, disc, low, st, stackMember); 
+
+			// Check if the subtree rooted with 'v' has a 
+			// connection to one of the ancestors of 'u' 
+			// Case 1 (per above discussion on Disc and Low value) 
+			low[u] = min(low[u], low[v]); 
+		} 
+
+		// Update low value of 'u' only of 'v' is still in stack 
+		// (i.e. it's a back edge, not cross edge). 
+		// Case 2 (per above discussion on Disc and Low value) 
+		else if (stackMember[v] == true) 
+			low[u] = min(low[u], disc[v]); 
+	} 
+
+	// head node found, pop the stack and print an SCC 
+	int w = 0; // To store stack extracted vertices 
+	if (low[u] == disc[u]) 
+	{ 
+		while (st->top() != u) 
+		{ 
+			w = (int) st->top(); 
+			cout << w << " "; 
+			stackMember[w] = false; 
+			st->pop(); 
+		} 
+		w = (int) st->top(); 
+		cout << w << "\n"; 
+		stackMember[w] = false; 
+		st->pop(); 
+	} 
+} 
+
+// The function to do DFS traversal. It uses SCCUtil() 
+void Graph::SCC() 
+{ 
+	int *disc = new int[V]; 
+	int *low = new int[V]; 
+	bool *stackMember = new bool[V]; 
+	stack<int> *st = new stack<int>(); 
+
+	// Initialize disc and low, and stackMember arrays 
+	for (int i = 0; i < V; i++) 
+	{ 
+		disc[i] = NIL; 
+		low[i] = NIL; 
+		stackMember[i] = false; 
+	} 
+
+	// Call the recursive helper function to find strongly 
+	// connected components in DFS tree with vertex 'i' 
+	for (int i = 0; i < V; i++) 
+		if (disc[i] == NIL) 
+			SCCUtil(i, disc, low, st, stackMember); 
+} 
+
+// Driver program to test above function 
+int main() 
+{ 
+	cout << "\nSCCs in first graph \n"; 
+	Graph g1(5); 
+	g1.addEdge(1, 0); 
+	g1.addEdge(0, 2); 
+	g1.addEdge(2, 1); 
+	g1.addEdge(0, 3); 
+	g1.addEdge(3, 4); 
+	g1.SCC(); 
+
+	cout << "\nSCCs in second graph \n"; 
+	Graph g2(4); 
+	g2.addEdge(0, 1); 
+	g2.addEdge(1, 2); 
+	g2.addEdge(2, 3); 
+	g2.SCC(); 
+
+	cout << "\nSCCs in third graph \n"; 
+	Graph g3(7); 
+	g3.addEdge(0, 1); 
+	g3.addEdge(1, 2); 
+	g3.addEdge(2, 0); 
+	g3.addEdge(1, 3); 
+	g3.addEdge(1, 4); 
+	g3.addEdge(1, 6); 
+	g3.addEdge(3, 5); 
+	g3.addEdge(4, 5); 
+	g3.SCC(); 
+
+	cout << "\nSCCs in fourth graph \n"; 
+	Graph g4(11); 
+	g4.addEdge(0,1);g4.addEdge(0,3); 
+	g4.addEdge(1,2);g4.addEdge(1,4); 
+	g4.addEdge(2,0);g4.addEdge(2,6); 
+	g4.addEdge(3,2); 
+	g4.addEdge(4,5);g4.addEdge(4,6); 
+	g4.addEdge(5,6);g4.addEdge(5,7);g4.addEdge(5,8);g4.addEdge(5,9); 
+	g4.addEdge(6,4); 
+	g4.addEdge(7,9); 
+	g4.addEdge(8,9); 
+	g4.addEdge(9,8); 
+	g4.SCC(); 
+
+	cout << "\nSCCs in fifth graph \n"; 
+	Graph g5(5); 
+	g5.addEdge(0,1); 
+	g5.addEdge(1,2); 
+	g5.addEdge(2,3); 
+	g5.addEdge(2,4); 
+	g5.addEdge(3,0); 
+	g5.addEdge(4,2); 
+	g5.SCC(); 
+
+	return 0; 
+}