class Solution {
public:
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
        vector<vector<int>> edge(numCourses);   // edge[i] 存 边
        vector<int> in(numCourses, 0);     // 入度
        for(int i = 0; i < prerequisites.size(); ++ i) {
            edge[prerequisites[i][1]].emplace_back(prerequisites[i][0]);
            ++ in[prerequisites[i][0]];
        }
        queue<int> que;     //
        for(int i = 0; i < numCourses; ++ i) {
            if(in[i] == 0) {
                que.push(i);
            }
        }
        while(!que.empty()) {
            int f = que.front(); que.pop();
            for(int i = 0; i < edge[f].size(); ++ i) {
                -- in[edge[f][i]];
                if(in[edge[f][i]] == 0) {
                    que.push(edge[f][i]);
                }
            }
        }
        for(int i = 0; i < numCourses; ++ i) {
            if(in[i] != 0) return false;
        }
        return true;
    }
};