Course scheduleII_2.cpp

class Solution {
public:
    vector<int> findOrder(int numCourses, vector<pair<int, int>>& prerequisites) {
        if (numCourses < 1)return {};
        vector<vector<int>> graph(numCourses, vector<int>());
        vector<int> in(numCourses, 0), res;
        for (auto p : prerequisites)
        {
            graph[p.second].push_back(p.first);
            in[p.first]++;
        }
        
        queue<int> q;
        for (int i = 0; i < numCourses; ++i)
        {
            if (!in[i])q.push(i);
        }
        
        while(!q.empty())
        {
            int curr = q.front();
            q.pop();
            res.push_back(curr);
            for (int i : graph[curr])
            {
                in[i]--;
                if (!in[i])q.push(i);
            }
        }
        
        return res.size() == numCourses? res: vector<int>();
    }
};