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.
解题思路：一刷没ac。 一个拓扑排序的思路是将所有课程看成是节点，依赖关系看作是边，先找到度为0的节点，从图中移除，然后继续找堵为0的节点，顺次移除。看是否能够满足全部移除。
public class Solution { public boolean canFinish(int numCourses, int[][] prerequisites) { int[][] matrix = new int[numCourses][numCourses]; int[] indegree = new int[numCourses]; int count = 0; for (int i = 0; i < prerequisites.length; i++) { int ready = prerequisites[i][0]; int pre = prerequisites[i][1]; if (matrix[pre][ready] == 0) indegree[ready]++; matrix[pre][ready] = 1; } LinkedList<Integer> queue = new LinkedList<Integer>(); for (int i = 0; i < numCourses; i++) { if (indegree[i] == 0) queue.offer(i); } while (!queue.isEmpty()) { int course = queue.poll(); count++; for (int i = 0; i < numCourses; i++) { if (matrix[course][i] != 0) { if (--indegree[i] == 0) queue.offer(i); } } } return count == numCourses; }}新闻热点
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