leetcode207.课程表（中等 拓扑排序）

解法：固定的拓扑排序模板 O(顶点数+边数)

class Solution {
public:
    bool canFinish(int numCourses, vector>& prerequisites) {
        //拓扑排序判断是否是有向无环图
        vector indegree(numCourses);
        vector> bian(numCourses);
        for (auto& each : prerequisites) {
            indegree[each[0]]++;
            bian[each[1]].push_back(each[0]);
        }
        queue q;
        for (int i = 0; i < numCourses; ++i) {
            if (!indegree[i]) q.push(i);
        }
        int cnt = 0;
        while (!q.empty()) {
            int frt = q.front();
            q.pop();
            cnt++;
            for (auto& each : bian[frt]) {
                indegree[each]--;
                if (!indegree[each]) q.push(each);
            }
        }
        if (cnt == numCourses) return true;
        return false;
    }
};