https://leetcode.com/problems/course-schedule/

Solution.java

import java.util.*;
import java.lang.*;

public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        return !hasCycle(numCourses, prerequisites);
    }

    private boolean hasCycle(int numCourses, int[][] prerequisites) {
        List<Integer>[] g = constructGraph(numCourses, prerequisites);
        Set<Integer> whiteSet = new HashSet<>();
        Set<Integer> greySet  = new HashSet<>();
        Set<Integer> blackSet = new HashSet<>();

        for (int i = 0; i < g.length; i++) {
            if (g[i] != null) {
                whiteSet.add(i);
            }
        }

        while (whiteSet.size() > 0) {
            Integer cur = whiteSet.iterator().next();
            if (dfs(cur, g, whiteSet, greySet, blackSet)) {
                return true;
            }
        }
        return false;
    }

    private boolean dfs(int edge,
                        List<Integer>[] g,
                        Set<Integer> whiteSet,
                        Set<Integer> greySet,
                        Set<Integer> blackSet) {
        moveVertex(edge, whiteSet, greySet);
        List<Integer> nodes;
        int n;
        if (g[edge] != null) {
            nodes = g[edge];
            for (int i = 0; i < nodes.size(); i++) {
                n = nodes.get(i);
                if (blackSet.contains(n)) {
                    continue;
                }
                if (greySet.contains(n)) {
                    return true;
                }
                if (dfs(n, g, whiteSet, greySet, blackSet)) {
                    return true;
                }
            }
        }
        moveVertex(edge, greySet, blackSet);
        return false;
    }

    private void moveVertex(int edge,
                            Set<Integer> s1,
                            Set<Integer> s2) {
        s1.remove(edge);
        s2.add(edge);
    }

    private List<Integer>[] constructGraph(int numCourses, int[][] edges) {
        int a;
        int b;
        List<Integer>[] g = (List<Integer>[])new ArrayList[numCourses];
        for (int i = 0; i < edges.length; i++) {
            a = edges[i][1];
            b = edges[i][0];
            if (g[a] == null) {
                g[a] = new ArrayList<>();
            }
            g[a].add(b);
        }
        return g;
    }
}