course shedule(略)

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.

public boolean canFinish(int numCourses, int[][] prerequisites) {    int[][] matrix = new int[numCourses][numCourses]; // i -> j    int[] indegree = new int[numCourses];    for (int i=0; i<prerequisites.length; i++) {        int ready = prerequisites[i][0];        int pre = prerequisites[i][1];        if (matrix[pre][ready] == 0)            indegree[ready]++; //duplicate case        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;}

http://blog.csdn.net/ljiabin/article/details/45846837