207. 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.

解题思路：一刷没ac。
一个拓扑排序的思路是将所有课程看成是节点，依赖关系看作是边，先找到度为0的节点，从图中移除，然后继续找堵为0的节点，顺次移除。看是否能够满足全部移除。

public class Solution {    public boolean canFinish(int numCourses, int[][] prerequisites) {        int[][] matrix = new int[numCourses][numCourses];        int[] indegree = new int[numCourses];        int count = 0;        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]++;            matrix[pre][ready] = 1;        }        LinkedList<Integer> queue = new LinkedList<Integer>();        for (int i = 0; i < numCourses; 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;    }}