(medium)LeetCode 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.

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

click to show more hints.

解法：拓扑排序topological sort

代码如下：

public class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
int [][]matrix=new int[numCourses][numCourses];
int [] indegree =new int[numCourses];
int len=prerequisites.length;
for(int i=0;i<len;i++){
int ready=prerequisites[i][0];
int pre=prerequisites[i][1];
if (matrix[pre][ready] == 0)//防止重复条件
indegree[ready]++;
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;
}
}

　　运行结果：