add program to solve CourseSolution problem

Author: rdahiya0213

LeetCode/CourseSchedule.java

@@ -0,0 +1,91 @@
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.*;
+
+/**
+ * Problem:
+ * Given N courses, labeled from 0 to N-1. Also given M pairs(X,Y) of prerequisites,
+ * where Y course must be finished before X.
+ * We need to tell if it is possible to finish all the courses.
+ *
+ * Approach: Topological Sorting, Cycle Detection
+ * Let's try to think of prerequisites pairs as a edge between two nodes in a graph.
+ * Now this problem becomes simple, we just need to find out if there is any cycle in a graph or not.
+ * If there is no cycle, then we can finish all the courses otherwise the answer is no.
+ */
+public class CourseSchedule {
+    public static boolean canFinish(int numCourses, int[][] prerequisites) {
+        // this is to keep track of visited nodes
+        boolean[] vis = new boolean[numCourses];
+
+        // initialize adjacent graph
+        List<List<Integer>> graph = new ArrayList<>();
+        for(int i = 0; i < numCourses; i++) {
+            graph.add(new ArrayList<>());
+        }
+
+        for(int i = 0; i < prerequisites.length; i++) {
+            graph.get(prerequisites[i][1]).add(prerequisites[i][0]);
+        }
+
+        // set default ans to true
+        // and we will only set it to false if and only if there is a cycle in the graph
+        boolean ans = true;
+
+        // this is use to keep track of visited nodes in the current chain
+        // Note: this is different from vis array
+        Set<Integer> currentVisNodes = new HashSet<>();
+        for(int i = 0; i < numCourses; i++) {
+            // if we haven't visited this node, then let's try to find if there is a cycle or not
+            if(!vis[i]) {
+                ans &= dfs(graph, i, vis, currentVisNodes);
+                currentVisNodes.clear();
+            }
+        }
+
+        return ans;
+    }
+
+    private static boolean dfs(List<List<Integer>> graph, int currentNode, boolean[] vis, Set<Integer> currentVisNodes) {
+        // set currentNode to visited
+        vis[currentNode] = true;
+        boolean possible = true;
+
+        // add this node to current chain
+        currentVisNodes.add(currentNode);
+        for(Integer nextNode: graph.get(currentNode)) {
+
+            // if we have found a node which we have already visited in the current chain then there is a loop
+            if(currentVisNodes.contains(nextNode)) {
+                possible = false;
+                break;
+            } else {
+                // otherwise if we haven't visited next node then let's keep doing dfs recursively
+                if(!vis[nextNode]) {
+                    possible &= dfs(graph, nextNode, vis, currentVisNodes);
+                }
+            }
+
+        }
+        // in the end after exploring all the adjacent nodes of this node, remove the current node from the chain
+        currentVisNodes.remove(Integer.valueOf(currentNode));
+        return possible;
+    }
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        int[] line = Arrays.stream(br.readLine().split(" ")).mapToInt(Integer::valueOf).toArray();
+        int n = line[0];
+        int m = line[1];
+
+        int[][] prerequisites = new int[m][2];
+        for(int i = 0; i < m; i++) {
+            line = Arrays.stream(br.readLine().split(" ")).mapToInt(Integer::valueOf).toArray();
+            prerequisites[i][0] = line[0];
+            prerequisites[i][1] = line[1];
+        }
+
+        System.out.println(canFinish(n, prerequisites));
+    }
+}