LeetCode "Course Schedule"

Topological sorting. This is my DFS solution. Please note that I used _rec to avoid duplicate checking.

class Solution {
public:
    map<int, int> rec;
    set<int> _rec;
    bool go(map<int, vector<int>> &m, int inx, int level)
    {
        rec[inx] = level;
        _rec.insert(inx);
        for (auto i : m[inx])
        {
            if (rec[i] != -1 && rec[i] < level) return false;

            bool b = go(m, i, level + 1);
            if (!b) return false;
        }
        rec[inx] = -1;
        return true;
    }
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) 
    {
        map<int, vector<int>> m;
        for (auto &r : prerequisites)
        {
            m[r.second].push_back(r.first);
        }
        for (int i = 0; i < numCourses; i++)
            rec[i] = -1;

        for (auto &r : m)
        {
            if(_rec.find(r.first) != _rec.end()) continue;
            
            bool ret = go(m, r.first, 0);
            if (!ret) return false;
        }
        return true;
    }
};