leetcode207.课程表

思路源自

【大厂程序员带你刷力扣】【LeetCode 207】课程表｜拓扑排序｜BFS

 这种先后顺序问题采用有向图存储，统计每个顶点的入度情况，然后采用拓扑排序算法看看是否能让所有顶点的入度全部为0，如果可以那么拓扑成功

class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        Map<Integer, ArrayList<Integer>> graph = new HashMap<>();//图存储
        int[] ingrees = new int[numCourses];//存储顶点入度情况
        //创建图和顶点入度情况
        for (int i = 0; i < numCourses; i++) {
            graph.put(i, new ArrayList<>());
        }
        for (int[] prerequisite : prerequisites) {
            int key = prerequisite[0];
            int value = prerequisite[1];
            ingrees[value]++;
            graph.get(key).add(value);
        }
        int count=0;//统计入度为0的个数
        Queue<Integer> queue = new LinkedList<>();
        for (int i = 0; i < ingrees.length; i++) {
            if (ingrees[i] == 0) {
                queue.offer(i);
                count++;
            }
        }
        while (!queue.isEmpty()) {
            Integer key = queue.poll();
            ArrayList<Integer> list = graph.get(key);
            for (Integer vertex : list) {
                ingrees[vertex]--;
                if (ingrees[vertex] == 0) {
                    count++;
                    queue.offer(vertex);
                }
            }
        }
        return numCourses == count;
    }
}