Course Schedule

There are a total of numCourses courses you have to take, labeled from 0 to numCourses-1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0. So it is possible.

Example 2:

Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0, and to take course 0 you should
             also have finished course 1. So it is impossible.

Constraints:

• The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.

• You may assume that there are no duplicate edges in the input prerequisites.

• 1 <= numCourses <= 10^5

class Solution {
    public static boolean canFinish(int numCourses, int[][] prerequisites) {
        // Graph creation
        List<Integer>[] graph = new ArrayList[numCourses];
        for (int i = 0; i < numCourses; i++)
            graph[i] = new ArrayList<>();
        // In-degree array
        int[] indegree = new int[numCourses];
        for (int[] edge : prerequisites) {
            graph[edge[0]].add(edge[1]);
            indegree[edge[1]]++;
        }
        Queue<Integer> q = new LinkedList<>();
        for (int i = 0; i < numCourses; i++)
            if (indegree[i] == 0)
                q.add(i);
        int covered = 0;
        while (q.size() != 0) {
            int course = q.poll();
            for (int neighbor : graph[course]) {
                indegree[neighbor]--;
                if (indegree[neighbor] == 0)
                    q.add(neighbor);
            }
            covered++;
        }
        return covered == numCourses;
    }
}