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;
    }