solves course schedule in java

anishLearnsToCode · anishLearnsToCode · commit eec695bad5d0 · 2022-05-08T14:54:38.000+02:00
diff --git a/README.md b/README.md
@@ -166,7 +166,7 @@
 | 204  | [Count Primes](https://leetcode.com/problems/count-primes)                                                                                                |                                       [![Java](assets/java.png)](src/CountPrimes.java) [![Python](assets/python.png)](python/count_primes.py)                                        |                                                                                                                                   |
 | 205  | [Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings)                                                                                    |                                 [![Java](assets/java.png)](src/IsomorphicStrings.java) [![Python](assets/python.png)](python/isomorphic_strings.py)                                  |                                                                                                                                   |
 | 206  | [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list)                                                                                  |                                 [![Java](assets/java.png)](src/ReverseLinkedList.java) [![Python](assets/python.png)](python/reverse_linked_list.py)                                 |                                                                                                                                   |
-| 207  | [Course Schedule](https://leetcode.com/problems/course-schedule)                                                                                          |                                                                                                                                                                                      |                                                                                                                                   |
+| 207  | [Course Schedule](https://leetcode.com/problems/course-schedule)                                                                                          |                                                                 [![Java](assets/java.png)](src/CourseSchedule.java)                                                                  |                                                                                                                                   |
 | 208  | [Implement Trie (Prefix Tree)](https://leetcode.com/problems/implement-trie-prefix-tree)                                                                  |                                                                                                                                                                                      |                                                                                                                                   |
 | 209  | [Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum)                                                                      |                                                                                                                                                                                      |                                                                                                                                   |
 | 210  | [Course Schedule II](https://leetcode.com/problems/course-schedule-ii)                                                                                    |                                                                                                                                                                                      |                                                                                                                                   |
diff --git a/src/CourseSchedule.java b/src/CourseSchedule.java
@@ -0,0 +1,62 @@
+// https://leetcode.com/problems/course-schedule
+// T: O(|pre-requisites| + numCourses)
+// S: O(numCourses)
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+public class CourseSchedule {
+    public boolean canFinish(int numCourses, int[][] prerequisites) {
+        final Map<Integer, List<Integer>> dependencyGraph = getDependencyGraph(prerequisites);
+        return !containsLoop(dependencyGraph, numCourses);
+    }
+
+    private boolean containsLoop(Map<Integer, List<Integer>> graph, int numCourses) {
+        Set<Integer> computed = new HashSet<>();
+        for (int i = 0 ; i < numCourses ; i++) {
+            if (graph.containsKey(i)) {
+                if (computed.contains(i)) continue;
+                if (containsLoop(graph, i, computed)) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+
+    private boolean containsLoop(Map<Integer, List<Integer>> graph, int vertex, Set<Integer> computed) {
+        return containsLoop(graph, vertex, computed, new HashSet<>());
+    }
+
+    private boolean containsLoop(Map<Integer, List<Integer>> graph, int vertex, Set<Integer> computed, Set<Integer> visited) {
+        if (visited.contains(vertex)) return true;
+        if (computed.contains(vertex)) return false;
+        visited.add(vertex);
+        computed.add(vertex);
+        if (graph.containsKey(vertex)) {
+            List<Integer> dependsOn = graph.get(vertex);
+            for (int dependency : dependsOn) {
+                if (containsLoop(graph, dependency, computed, visited)) {
+                    return true;
+                }
+            }
+        }
+        visited.remove(vertex);
+        return false;
+    }
+
+    private Map<Integer, List<Integer>> getDependencyGraph(int[][] prerequisites) {
+        final Map<Integer, List<Integer>> dependencyGraph = new HashMap<>();
+        for (int[] prerequisite : prerequisites) {
+            if (!dependencyGraph.containsKey(prerequisite[0])) {
+                dependencyGraph.put(prerequisite[0], new ArrayList<>());
+            }
+            dependencyGraph.get(prerequisite[0]).add(prerequisite[1]);
+        }
+        return dependencyGraph;
+    }
+}
