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package leetcode;

import java.util.ArrayList;

import java.util.LinkedList;

import java.util.List;

import java.util.Queue;

/**

* Project Name : Leetcode

* Package Name : leetcode

* File Name : CourseSchedule

* Creator : Edward

* Date : Nov, 2017

* Description : 207. Course Schedule

*/

public class CourseSchedule {

/**

* 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.

3- 0 - 1

\ /

2

入度 = 0

0 : 1

1 : 1

2 : 1

queue : 2

pre : 2

res = 3

time : O(V + E)

space : O(n)

* @param numCourses

* @param prerequisites

* @return

*/

// BFS

public boolean canFinish(int numCourses, int[][] prerequisites) {

int[] indegree = new int[numCourses];

int res = numCourses;

for (int[] pair : prerequisites) {

indegree[pair[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 pre = queue.poll();

res--;

for (int[] pair : prerequisites) {

if (pair[1] == pre) {

indegree[pair[0]]--;

if (indegree[pair[0]] == 0) {

queue.offer(pair[0]);

}

}

}

}

return res == 0;

}

}