Create TopologicalSort_DFS.java

Author: 17tanya

GraphsDFS&BFS/TopologicalSort_DFS.java

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+//Reference - https://www.geeksforgeeks.org/topological-sorting/
+
+/*Algorithms - 
+1. Maintain a visited set and a stack to record the order of vertices
+2. Starting from a vertex loop through all the vertices
+  2.1 check if the vertex is present in the visited set
+    (i) vertex in visited set ==> continue the loop
+    (ii) vertex not in visited set ==> explore children of the current vertex recursivley ==> in a Depth First manner
+         Exploring a vertex implies visiting it's children. When there are no children to explore push the vertex to the stack  
+*/
+
+class topologicalSort{
+    int vertices;
+    ArrayList<Integer> adjList[]; //adjacency list
+    
+    topologicalSort(int n){
+        vertices = n;
+        adjList = new ArrayList[n];
+        for(int i=0;i<n;i++) adjList[i] = new ArrayList<Integer>();
+    }
+    
+    void topo(){
+        boolean viisted[] = new boolean[vertices]; //keep track of visited vertices
+        Arrays.fill(visited,false);
+        
+        Stack<Integer> stk = new Stack<Integer>(); //keep track of the order
+        
+        for(int i=0;i<vertices;i++){
+            if(!visited[i]) topoUtil(i,visited,stk);
+        }
+        
+        //print out the topological sort
+        while(!stk.isEmpty()){
+            System.out.println(stk.pop());
+        }
+    }
+    
+    void topoUtil(int v,boolean visited[],Stack<Integer> stk){
+        visited[v] = true;
+        ArrayList<Integer> neighbours = adjList[v];
+        
+        for(int i=0 ; i<neighbours.size() ; i++){ //exploring neighbours
+            int neighbour = neighbours.get(i);
+            if(!visited[neighbour]) topoUtil(neighbour,visited,stk); //exploring neighbours in Depth First Manner
+        }
+        stk.push(v);
+    }
+    
+}