Course Schedule

There are a total of n courses you have to take, labeled from 0 to n - 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?

For example:

2, [[1,0]]

There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.

2, [[1,0],[0,1]]

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.

Note:

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

Code:

public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        int[][] matrix = new int[numCourses][numCourses]; // i -> j
        int[] indegree = new int[numCourses];

        for (int i=0; i<prerequisites.length; i++) {
            int ready = prerequisites[i][0];
            int pre = prerequisites[i][1];
            if (matrix[pre][ready] == 0)
                indegree[ready]++; //duplicate case
            matrix[pre][ready] = 1;
        }

        int count = 0;
        Queue<Integer> queue = new LinkedList();
        for (int i=0; i<indegree.length; i++) {
            if (indegree[i] == 0) queue.offer(i);
        }
        while (!queue.isEmpty()) {
            int course = queue.poll();
            count++;
            for (int i=0; i<numCourses; i++) {
                if (matrix[course][i] != 0) {
                    if (--indegree[i] == 0)
                        queue.offer(i);
                }
            }
        }
        return count == numCourses;
    }
}