leetcode 207: Course Schedule，leetcodeschedule

leetcode 207: Course Schedule，leetcodeschedule
Course ScheduleTotal Accepted: 3707 Total Submissions: 18102

There are a total of n courses you have to take, labeled from 0 ton - 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 abouthow a graph is represented.

[思路]

此問題等價於 圖(or forest)中有無環的存在.

使用topological sorting, 成功sort後,如果prerequisite沒有空,則沒有環.

[CODE]

public class Solution {    // [4, 3] [3,2] [2,1]    //init check.    public boolean canFinish(int numCourses, int[][] prerequisites) {        Set<int[]> pre = new HashSet<int[]>();        for(int[] e : prerequisites) {            pre.add(e);        }                int n = pre.size();        if(n > numCourses*(numCourses-1)/2) return false;                while(!getEnds(pre).isEmpty() ){};        return pre.isEmpty();    }    // [1,2] [2, 3] [3,4]    private Set<Integer> getEnds(Set<int[]> pre) {        Set<Integer> set = new HashSet<Integer>();        for(int[] arr : pre) {            set.add(arr[0]);        }                for(int[] arr: pre) {            set.remove(arr[1]);        }                Iterator<int[]> iter = pre.iterator();          while(iter.hasNext() ) {            int[] arr = iter.next();            if(set.contains(arr[0]) ) iter.remove();        }        return set;    }}