add: Prerequisites

Author: Jer3myYu

java/Graphs/Prerequisites.java

@@ -0,0 +1,47 @@
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Queue;
+
+public class Prerequisites {
+    public boolean prerequisites(int n, int[][] prerequisites) {
+        Map<Integer, List<Integer>> graph = new HashMap<>();
+        int[] inDegrees = new int[n];
+        // Represent the graph as an adjacency list and record the in-
+        // degree of each course.
+        for (int[] edge : prerequisites) {
+            int prerequisite = edge[0];
+            int course = edge[1];
+            graph.putIfAbsent(prerequisite, new ArrayList<>());
+            graph.get(prerequisite).add(course);
+            inDegrees[course]++;
+        }
+        Queue<Integer> queue = new ArrayDeque<>();
+        // Add all courses with an in-degree of 0 to the queue.
+        for (int i = 0; i < n; i++) {
+            if (inDegrees[i] == 0) {
+                queue.offer(i);
+            }
+        }
+        int enrolledCourses = 0;
+        // Perform topological sort.
+        while (!queue.isEmpty()) {
+            int node = queue.poll();
+            enrolledCourses++;
+            if (graph.containsKey(node)) {
+                for (int neighbor : graph.get(node)) {
+                    inDegrees[neighbor]--;
+                    // If the in-degree of a neighboring course becomes 0, add 
+                    // it to the queue.
+                    if (inDegrees[neighbor] == 0) {
+                        queue.offer(neighbor);
+                    }
+                }
+            }
+        }
+        // Return true if we've successfully enrolled in all courses.
+        return enrolledCourses == n;
+    }
+}