(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：