Create Course_Schedule.java

Author: 17tanya

GraphsDFS&BFS/Course_Schedule.java

@@ -0,0 +1,59 @@
+//Leetcode 207. Course Schedule
+//Question - https://leetcode.com/problems/course-schedule/
+//Used Kahn's Topological Sort
+
+class Solution {
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        
+        /*Input Description - 
+            numCourses ==> number of nodes in the graph
+            prerequisites ==> 1. each row is a pair of vertices
+                              2. represents edges in the graph
+                              3. edge direction is prerequisites[row][1] --> prerequisites[row][0]
+                              4. NOTE - (prerequisites.length != numCourses) because (prerequisites.length==number of edges) & (numCourses==vertices)
+        */
+        
+        
+        int inDegree[] = new int[numCourses]; //keep track of indegrees of all nodes
+        Arrays.fill(inDegree,0);
+        
+        for(int i=0;i<prerequisites.length;i++){
+            inDegree[prerequisites[i][0]]++;
+        }
+        
+        Queue<Integer> inDegree0 = new LinkedList<>(); //to add vertices of inDegree 0 
+        for(int i=0;i<numCourses;i++){
+            if(inDegree[i]==0) inDegree0.add(i);
+        }
+        
+        int vertexCnt = 0; //needed to check if cycle present or not
+        
+        /*for a cyclic component in a garph the inDegree will never be 0 ==> vertices in that component will never be added in the queue ==>
+        vertexCnt will not increment for those vertices ==> (vertexCnt != numCourses)
+        */
+        
+        while(!inDegree0.isEmpty()){
+            int curr = inDegree0.poll();
+            ArrayList<Integer> neighbours = findNeighbours(prerequisites,curr); //removing this vertex from graph
+            
+            for(int neighbour : neighbours){
+            //reflecting the change after removing the vertex
+                if(--inDegree[neighbour]==0) inDegree0.add(neighbour);
+            }
+            
+            vertexCnt++;
+        }
+        
+        if(vertexCnt==numCourses) return true; //no cycles in graph
+        else return false; //cycles in garph
+        
+    }
+    
+    public ArrayList<Integer> findNeighbours(int[][] prerequisites,int v){
+        ArrayList<Integer> neighbour = new ArrayList<>();
+        for(int i=0 ; i<prerequisites.length ; i++){
+            if(prerequisites[i][1]==v) neighbour.add(prerequisites[i][0]);
+        }
+        return neighbour;
+    }
+}