course shed ii

course-schedule-ii/Solution.java

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+public class Solution {
+
+    static class Vertex {
+
+        int id;
+
+        Vertex(int id){
+            this.id = id;
+        }
+
+        Set<Integer> in  = new HashSet<>();
+        Set<Integer> out = new HashSet<>();
+
+        boolean isSink(){
+            return out.isEmpty();
+        }
+    }
+
+    Vertex safe(Vertex[] G, int id){
+        if(G[id] == null){
+            G[id] = new Vertex(id);
+        }
+
+        return G[id];
+    }
+
+    public int[] findOrder(int numCourses, int[][] prerequisites) {
+        Vertex[] G = new Vertex[numCourses];
+
+        for(int[] p : prerequisites){
+            safe(G, p[0]).out.add(p[1]);
+            safe(G, p[1]).in.add(p[0]);
+        }
+
+        Set<Vertex> S = Arrays.stream(G)
+                            .filter(v -> v != null)
+                            .collect(Collectors.toSet());
+
+
+        LinkedHashSet<Integer> order = new LinkedHashSet<>(numCourses);
+
+        loop:
+        while(!S.isEmpty()){
+
+            for(Vertex v : S){
+                if(v.isSink()){
+                    order.add(v.id);
+
+                    S.remove(v);
+
+                    for(int i : v.in){
+                        G[i].out.remove(v.id);
+                    }
+
+                    continue loop;
+                }
+            }
+
+            return new int[]{};
+        }
+
+        // fill courses not in G
+        order.addAll(IntStream.range(0, numCourses).boxed().collect(Collectors.toSet()));
+
+        return order.stream()
+                .mapToInt(i -> i)
+                .toArray();
+    }
+}

course-schedule-ii/index.md

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+---
+layout: solution
+title: Course Schedule II
+date: 2015-05-14 00:31:26+08:00
+leetcode_id: 210
+---
+{% include_relative README.md %}