topo.cpp

//Topological sorting for Directed Acyclic Graph (DAG) is a linear ordering of vertices such that for every directed edge uv, vertex u comes before v in the ordering. Topological Sorting for a graph is not possible if the graph is not a DAG.
#include <iostream>
#include <list>
#include <stack>
using namespace std;

// Class to represent a graph
class Graph {
	int V; // No. of vertices
	// Pointer to an array containing adjacency listsList
	list<int>* adj;

	// A function used by topologicalSort
	void topologicalSortUtil(int v, bool visited[], stack<int>& Stack);

public:
	Graph(int V); // Constructor

	// function to add an edge to graph
	void addEdge(int v, int w);

	// prints a Topological Sort of the complete graph
	void topologicalSort();
};

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.
}

// A recursive function used by topologicalSort
void Graph::topologicalSortUtil(int v, bool visited[],
								stack<int>& Stack)
{
	// Mark the current node as visited.
	visited[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])
			topologicalSortUtil(*i, visited, Stack);

	// Push current vertex to stack which stores result
	Stack.push(v);
}

// The function to do Topological Sort. It uses recursive
// topologicalSortUtil()
void Graph::topologicalSort()
{
	stack<int> Stack;

	// Mark all the vertices as not visited
	bool* visited = new bool[V];
	for (int i = 0; i < V; i++)
		visited[i] = false;

	// Call the recursive helper function to store Topological
	// Sort starting from all vertices one by one
	for (int i = 0; i < V; i++)
		if (visited[i] == false)
			topologicalSortUtil(i, visited, Stack);

	// Print contents of stack
	while (Stack.empty() == false) {
		cout << Stack.top() << " ";
		Stack.pop();
	}
}

// Driver program to test above functions
int main()
{
	// Create a graph given in the above diagram
	Graph g(6);
	g.addEdge(5, 2);
	g.addEdge(5, 0);
	g.addEdge(4, 0);
	g.addEdge(4, 1);
	g.addEdge(2, 3);
	g.addEdge(3, 1);

	cout << "Following is a Topological Sort of the given graph n";
	g.topologicalSort();

	return 0;
}