【Leetcode】Course Schedule

题目链接：
题目：https://leetcode.com/problems/course-schedule/

题目：
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.

思路：
拓扑排序，计算无环结点个数，

算法：

    public boolean canFinish(int numCourses, int[][] prerequisites) {        int preCount[] = new int[numCourses];        Queue<Integer> notPre = new LinkedList<Integer>();        // 统计每个结点作为前驱的次数        for (int i = 0; i < prerequisites.length; i++) {            preCount[prerequisites[i][0]]++;        }        // 记录出度为0的结点        for (int i = 0; i < preCount.length; i++) {            if (preCount[i] == 0) {                notPre.offer(i);            }        }        int num = notPre.size();// 可以拓扑排序的节点个数        while (!notPre.isEmpty()) {//BFS            int node = notPre.poll();            for (int i = 0; i < prerequisites.length; i++) {                if (prerequisites[i][1] == node) {                    preCount[prerequisites[i][0]]--;//删除和node有关的边                    if (preCount[prerequisites[i][0]] == 0) {//如果出度为0                        num++;                        notPre.offer(prerequisites[i][0]);                    }                }            }        }        return num == numCourses;    }