Level：
Medium
题目描述：
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?
Example 1:
Input: 2, [[1,0]] Output: trueExplanation: 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: 2, [[1,0],[0,1]]Output: falseExplanation: 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 .
- You may assume that there are no duplicate edges in the input prerequisites.
思路分析：
这题是拓扑排序一道应用，首先，我们应该统计每个点的入度，然后根据拓扑排序的规则，先删去入度为0的点，直到最后点的个数为0，则是正确的排课。
代码：
public class Solution{ public boolean canFinish(int coursenum,int[][]prerequisites){ int []indegree=new int [coursenum];//记录每门课的入度。 for(int []pair:prerequisites){ indegree[pair[0]]++; //统计每门课的入度 } Queue q=new LinkedList<>(); //存放入度为0的课程，准备删除 for(int i=0;i