CourseSchedule.java

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        List<List<Integer>> adj = new ArrayList<>();
        for (int i = 0; i < numCourses; i++) {
            adj.add(new ArrayList<>());
        }
        
        for (int i = 0; i < prerequisites.length; i++) {
            adj.get(prerequisites[i][0]).add(prerequisites[i][1]);
        }
        
        int[] visited = new int[numCourses];
        for (int i = 0; i < numCourses; i++) {
            if (visited[i] == 0) {
                if (isCyclic(adj, visited, i)) {
                    return false;
                }
            }
        }
        return true;
    }
    
    private boolean isCyclic(List<List<Integer>> adj, int[] visited, int curr) {
        if (visited[curr] == 2) {
            return true;
        }
        
        visited[curr] = 2;
        for (int i = 0; i < adj.get(curr).size(); i++) {
            if (visited[adj.get(curr).get(i)] != 1) {
                if (isCyclic(adj, visited, adj.get(curr).get(i))) {
                    return true;
                }
            }
        }
        visited[curr] = 1;
        return false;
    }
}