207. Course Schedule(gragh)

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.
前置课程问题，典型的拓扑排序
public class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
int count=numCourses;

List<List<Integer>> gragh = new ArrayList<List<Integer>>();
for(int i=0;i<numCourses;i++)
{
List<Integer> temp = new LinkedList<Integer>();
gragh.add(temp);
}
//存储没门课程还需的前置课程数
int[] courses = new int[numCourses];
for(int i=0;i<prerequisites.length;i++)
{
//保存修完prerequisites[i][1]后能修得课程prerequisites[i][0]
gragh.get(prerequisites[i][1]).add(prerequisites[i][0]);
courses[prerequisites[i][0]]++;
}
Queue< Integer> queue = new LinkedList<Integer>();
for(int i=0;i<numCourses;i++)
{
if(courses[i]==0)
{
queue.offer(i);
}
}

while(!queue.isEmpty())
{
int cur = queue.poll();
//遍历当前课程得后续课程，并将后续课程的还需修前置课程减一
for(int temp :gragh.get(cur))
{
if(--courses[temp]==0)
{
queue.offer(temp);
}
}
count--;
}

return count==0;
}
}