Update course_schedule.cpp

Author: avidLearnerInProgress

course_schedule.cpp

@@ -1,35 +1,69 @@
-class Solution {
-public:
-    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
+// class Solution {
+// public:
+//     bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
 
-        vector<vector<int>> graph(numCourses);
-        vector<int> indegree(numCourses,0);
+//         vector<vector<int>> graph(numCourses);
+//         vector<int> indegree(numCourses,0);
 
-        for(auto edge : prerequisites)
-        {
-            graph[edge[1]].push_back(edge[0]);
-            indegree[edge[0]]++;
-        }
+//         for(auto edge : prerequisites)
+//         {
+//             graph[edge[1]].push_back(edge[0]);
+//             indegree[edge[0]]++;
+//         }
 
-        queue<int> Q;
+//         queue<int> Q;
 
-        for(int i = 0;i < numCourses;i++)
-            if(indegree[i] == 0)
-                Q.push(i);
+//         for(int i = 0;i < numCourses;i++)
+//             if(indegree[i] == 0)
+//                 Q.push(i);
 
-        int counter = 0;
-        while(!Q.empty())
-        {
-            int u = Q.front();
-            Q.pop();
+//         int counter = 0;
+//         while(!Q.empty())
+//         {
+//             int u = Q.front();
+//             Q.pop();
 
-            counter++;
-            for(auto v : graph[u])
-            {
-                if(--indegree[v] == 0)
-                    Q.push(v);
+//             counter++;
+//             for(auto v : graph[u])
+//             {
+//                 if(--indegree[v] == 0)
+//                     Q.push(v);
+//             }
+//         }
+//         return counter == numCourses;
+//     }
+// };
+
+class Solution {
+public:
+    bool isCyclic(vector<int> pre[], vector<int> &visited, int node) {
+        
+        if(visited[node] == 1) return true; //in-process already..
+        
+        if(visited[node] == 0) { //processing unprocessed node..
+            visited[node] = 1;
+            for(auto child : pre[node]) {
+                if(isCyclic(pre, visited, child))
+                    return true;
+            }
+        }        
+        visited[node] = 2;
+        return false;
+    }
+    
+    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
+        
+        vector<int> prereq[numCourses];
+        for(auto edge : prerequisites) {
+            prereq[edge[1]].push_back(edge[0]);
+        }
+        
+        vector<int> visited(numCourses, 0);
+        for(int i=0; i<numCourses; i++) {
+            if(isCyclic(prereq, visited, i)) {
+                return false;
             }
         }
-        return counter == numCourses;
+        return true;
     }
-};
+};