[leetcode-210]course scheduleII(java)

问题描述：
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, return the ordering of courses you should take to finish all courses.

There may be multiple correct orders, you just need to return one of them. If it is impossible to finish all courses, return an empty array.

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 the correct course order is [0,1]

4, [[1,0],[2,0],[3,1],[3,2]]
There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. So one correct course order is [0,1,2,3]. Another correct ordering is[0,2,1,3].

这道题恶心了我好久，我想过广搜，想过抽象成图，然后搜索，但是无论哪种都会出现超时。借鉴了网友的思想，很巧妙，没有用什么复杂的结构。只是记住一点：当要选择课程时，该课程必须没有依赖其他的课程。看上去很简单，但是就是这么简单的思想。

代码如下：４９６ｍｓ

public class Solution {         public int[] findOrder(int numCourses, int[][] prerequisites) {            int[] res = new int[numCourses];            int[] preNums = new int[numCourses];//preNums[j] store the num which before course j have to taken            //create numCourse sets which every one store the course set which must taken after course j            ArrayList<HashSet<Integer>> sets = new ArrayList<HashSet<Integer>>();            //post store the course j which have to before course j            HashMap<Integer, HashSet<Integer>> post = new HashMap<Integer, HashSet<Integer>>();            for(int i = 0;i<numCourses;i++){                sets.add(new HashSet<Integer>());            }            int preLen = prerequisites.length;            //init sets            for(int i = 0;i<preLen;i++){                if(prerequisites[i].length!=2)                    return res;                int key = prerequisites[i][0];                int value = prerequisites[i][1];                sets.get(value).add(key);                if(post.containsKey(key))                    post.get(key).add(value);                else {                    HashSet<Integer> tmpset = new HashSet<Integer>();                    tmpset.add(value);                    post.put(key, tmpset);                }            }            //init preNums            for(int i = 0;i<numCourses;i++){                if(post.containsKey(i))                    preNums[i] = post.get(i).size();                else                    preNums[i] = 0;            }            int index = 0;            int i;            while(index < numCourses){                //if course j has no pre course,then select it                //if no such course,which will be a circle                for(i = 0;i<numCourses;i++)                    if(preNums[i]==0){                        preNums[i] = -1;                        break;                    }                if(i == numCourses)                    return new int[0];                res[index++] = i;                //select the list which relay on the course j                HashSet<Integer> tmpSet = sets.get(i);                Iterator<Integer> iterator = tmpSet.iterator();                while(iterator.hasNext())                    preNums[iterator.next()]--;            }            return res;      }}