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| 1 | +// https://leetcode.com/problems/course-schedule |
| 2 | +// T: O(|pre-requisites| + numCourses) |
| 3 | +// S: O(numCourses) |
| 4 | + |
| 5 | +import java.util.ArrayList; |
| 6 | +import java.util.HashMap; |
| 7 | +import java.util.HashSet; |
| 8 | +import java.util.List; |
| 9 | +import java.util.Map; |
| 10 | +import java.util.Set; |
| 11 | + |
| 12 | +public class CourseSchedule { |
| 13 | + public boolean canFinish(int numCourses, int[][] prerequisites) { |
| 14 | + final Map<Integer, List<Integer>> dependencyGraph = getDependencyGraph(prerequisites); |
| 15 | + return !containsLoop(dependencyGraph, numCourses); |
| 16 | + } |
| 17 | + |
| 18 | + private boolean containsLoop(Map<Integer, List<Integer>> graph, int numCourses) { |
| 19 | + Set<Integer> computed = new HashSet<>(); |
| 20 | + for (int i = 0 ; i < numCourses ; i++) { |
| 21 | + if (graph.containsKey(i)) { |
| 22 | + if (computed.contains(i)) continue; |
| 23 | + if (containsLoop(graph, i, computed)) { |
| 24 | + return true; |
| 25 | + } |
| 26 | + } |
| 27 | + } |
| 28 | + return false; |
| 29 | + } |
| 30 | + |
| 31 | + private boolean containsLoop(Map<Integer, List<Integer>> graph, int vertex, Set<Integer> computed) { |
| 32 | + return containsLoop(graph, vertex, computed, new HashSet<>()); |
| 33 | + } |
| 34 | + |
| 35 | + private boolean containsLoop(Map<Integer, List<Integer>> graph, int vertex, Set<Integer> computed, Set<Integer> visited) { |
| 36 | + if (visited.contains(vertex)) return true; |
| 37 | + if (computed.contains(vertex)) return false; |
| 38 | + visited.add(vertex); |
| 39 | + computed.add(vertex); |
| 40 | + if (graph.containsKey(vertex)) { |
| 41 | + List<Integer> dependsOn = graph.get(vertex); |
| 42 | + for (int dependency : dependsOn) { |
| 43 | + if (containsLoop(graph, dependency, computed, visited)) { |
| 44 | + return true; |
| 45 | + } |
| 46 | + } |
| 47 | + } |
| 48 | + visited.remove(vertex); |
| 49 | + return false; |
| 50 | + } |
| 51 | + |
| 52 | + private Map<Integer, List<Integer>> getDependencyGraph(int[][] prerequisites) { |
| 53 | + final Map<Integer, List<Integer>> dependencyGraph = new HashMap<>(); |
| 54 | + for (int[] prerequisite : prerequisites) { |
| 55 | + if (!dependencyGraph.containsKey(prerequisite[0])) { |
| 56 | + dependencyGraph.put(prerequisite[0], new ArrayList<>()); |
| 57 | + } |
| 58 | + dependencyGraph.get(prerequisite[0]).add(prerequisite[1]); |
| 59 | + } |
| 60 | + return dependencyGraph; |
| 61 | + } |
| 62 | +} |
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