Leetcode 207 课程表 (拓扑排序，判断有向图环)

class Solution {
public:
    static const int maxn = 10000;
    vector<int> gra[maxn];
    int co = 0;
    int count[maxn] = { 0 };  //入度
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
        queue<int> Q;
        for (int i = 0;i < prerequisites.size();i++) {
            gra[prerequisites[i][1]].push_back(prerequisites[i][0]);
            count[prerequisites[i][0]]++;
        }
        int co = 0;
        for (int i = 0;i < numCourses;i++)
            if (count[i]==0) {
                Q.push(i);
            }
        while (!Q.empty()) {
            int cur = Q.front();
            Q.pop();
            co++;
            if (co > numCourses)break;
            for (int i = 0;i < gra[cur].size();i++) {
                count[gra[cur][i]]--;
                if (count[gra[cur][i]] == 0) {
                    Q.push(gra[cur][i]);
                }
            }
            gra[cur].clear();
        }
        if (co == numCourses)return true;
        else return false;
    }
};