[LeetCode 207] Course Schedule

There are a total ofncourses you have to take, labeled from0ton – 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 prerequisitepairs, 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 bya list of edges, not adjacency matrices. Read more abouthow a graph is represented.

topological sort, in a directed graph, every time found node without pre-node, then remove all its edges. until iterate all nodes or found cycle.

public boolean canFinish(int numCourses, int[][] prerequisites) {List<HashSet> preCourse = new ArrayList<HashSet>();for(int i=0;i<numCourses;i++){preCourse.add(new HashSet<Integer>());}for(int i=0;i<prerequisites.length;i++){preCourse.get(prerequisites[i][1]).add(prerequisites[i][0]);}int[] prenums = new int[numCourses];for(int i=0;i<numCourses;i++){Set set = preCourse.get(i);Iterator<Integer> iterator = set.iterator();while(iterator.hasNext()){prenums[iterator.next()]++;}}for(int i=0;i<numCourses;i++){int j=0;for(;j<numCourses;j++){if(prenums[j]==0) break;}if(j == numCourses) return false;prenums[j] = -1;Set set = preCourse.get(j);Iterator<Integer> iterator = set.iterator();while(iterator.hasNext()){prenums[iterator.next()]–;}}return true;}

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