[LeetCode 207] Course Schedule
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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.
Note:
The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how 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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