class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<vector<int>> graph(numCourses, vector<int>());
        vector<int> inDegree(numCourses); // 入度为 0 时顶点可以删除
        vector<int> res;
        queue<int> q; // 存储入度为 0 的顶点
        int cnt = 0; // 放入 res 的元素个数

        for(auto &p : prerequisites) {
            graph[p.second].push_back(p.first);
            inDegree[p.first]++;
        }

        for(int i = 0; i < numCourses; i++)
            if(inDegree[i] == 0)
                q.push(i);

        while(!q.empty()){
            int cur = q.front();
            q.pop();

            res.push_back(cur);
            cnt++;

            for(auto j : graph[cur]){
                inDegree[j]--;
                if(inDegree[j] == 0)
                    q.push(j);
            }
        }

        if(cnt < numCourses) {
            return vector<int>();
        }
        return res;
    }
};