bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {

        vector<set<int>> matrix(numCourses);

        for(int i=0; i<prerequisites.size(); i++)

            matrix[prerequisites[i].second].insert(prerequisites[i].first);  

        vector<int> degree(numCourses, 0);

        for(int i=0; i<numCourses; i++)

            for(auto it:matrix[i])

                ++degree[it];  

        for(int i=0; i<numCourses; i++)

        {

            int j;

            for(j=0; j<numCourses && degree[j]!=0; ++j);  

            if(j==numCourses) return false;  

            degree[j] = -1;  

            for(auto it:matrix[j])

                --degree[it];

        }  

        return true;

    }