class Solution {public:    vector<vector<int>> ret;    void go(int currMaxN, int currK, int k, unordered_set<int> set, vector<int> currRet)    {        if (currK == k)        {            ret.push_back(currRet);            return;        }        auto it = set.begin();        for (int i : set)        {            if (i > currMaxN)            {                vector<int> thisRet = currRet;                thisRet.push_back(i);                unordered_set<int> thisSet = set;                thisSet.erase(i);                go(i, currK + 1, k, thisSet, thisRet);            }        }    }    vector<vector<int> > combine(int n, int k) {        if (k > 0)        {            unordered_set<int> set;            for (int i = 1; i <= n; i++) set.insert(i);            for (int i = 1; i <= n - k + 1; i++)            {                unordered_set<int> currSet = set;                currSet.erase(i);                vector<int> v; v.push_back(i);                go(i, 1, k, currSet, v);            }        }        return ret;    }};