class Solution {public:    vector
    
     
      > combine(int n, int k) {        vector
      
       
        > res;        vector
        
          out;        helper(n, k, 1, out, res);        return res;    }    void helper(int n, int k, int level, vector
         
          & out, vector
          
           
            >& res) { if (out.size() == k) {res.push_back(out); return;} for (int i = level; i <= n; ++i) { out.push_back(i); helper(n, k, i + 1, out, res); out.pop_back(); } }};