Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

 

Code:

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