Graphs

Graphs/TopologicalSort.cpp

@@ -0,0 +1,75 @@
+// Topological Sort 
+// O (V+E)
+
+#include <bits/stdc++.h>
+using namespace std;
+
+void init(){
+  #ifndef ONLINE_JUDGE
+  freopen("input.txt", "r", stdin) ;
+  freopen("output.txt", "w", stdout) ;
+  #endif
+}
+
+bool dfs(int v, vector<vector<int>>& graph, vector<bool>& visited, vector<bool>& recStack, stack<int>& topologicalOrder){
+	visited[v]=true;
+	recStack[v]=true;
+	for(int child: graph[v]){
+		if(recStack[child])
+			return false;
+		if(visited[child]==false){
+			if(dfs(child,graph,visited,recStack,topologicalOrder)==false)
+				return false;
+		}
+	}
+	topologicalOrder.push(v);
+	recStack[v]=false;
+	return true;
+}
+vector<int> findTopologicalOrder(int numCourses, vector<vector<int>>& graph) {
+
+	// traverse the graph and find topological sort
+	stack<int> topologicalOrder;
+	vector<bool> visited(numCourses);// for visited node
+	vector<bool> recStack(numCourses);// for rec stack to check if there is back-edge
+	for(int i=0;i<numCourses;i++){
+		visited[i]=false;
+	}
+	// pick any non-visited vertex;
+	vector<int> ans;
+	for(int i=0;i<numCourses;i++){
+		if(visited[i]==false){
+			if(dfs(i,graph,visited,recStack,topologicalOrder)==false){// means there is backedge
+				return ans;
+			}
+		}
+	}
+
+	if(topologicalOrder.size()!=numCourses)
+		return ans;
+	while(topologicalOrder.size()!=0){
+		ans.push_back(topologicalOrder.top());
+		topologicalOrder.pop();
+	}
+	return ans;
+}
+
+int main(){
+    int t=1;
+    while(t--){
+		int n,m,a,b;
+		cout<<"Enter Nodes and Edges Count ";
+		cin>>n>>m;
+		//create graph
+		vector<vector<int>> graph(n);
+		for(int i=0;i<m;i++){
+			cin>>a>>b;
+			graph[a].push_back(b);
+		}
+        vector<int> ans=findTopologicalOrder(n,graph);
+        cout<<"The topological sort is as follows\n";
+		for(int i:ans)
+			cout<<i<<" ";
+		cout<<"\n";
+	}
+}