[leetcode]Topological Sort-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:

1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
2. You may assume that there are no duplicate edges in the input prerequisites.

 

click to show more hints.

public class Solution {  
    public boolean canFinish(int numCourses, int[][] prerequisites) {  
        int[] pre = new int[numCourses];  
        List<Integer>[] satisfies = new List[numCourses];  
        for(int i=0; i<numCourses; i++) satisfies[i] = new ArrayList<>();  
        for(int i=0; i<prerequisites.length; i++) {  
            satisfies[prerequisites[i][1]].add(prerequisites[i][0]);  
            pre[prerequisites[i][0]] ++;  
        }  
        int finish = 0;  
        LinkedList<Integer> queue = new LinkedList<>();  
        for(int i=0; i<numCourses; i++) {  
            if (pre[i] == 0) queue.add(i);  
        }  
        while (!queue.isEmpty()) {  
            int course = queue.remove();  
            finish ++;  
            if (satisfies[course] == null) continue;  
            for(int c: satisfies[course]) {  
                pre[c] --;  
                if (pre[c] == 0) queue.add(c);  
            }  
        }  
        return finish == numCourses;  
    }  
}