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],
]

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> tmp;
        helper(res,tmp,n,k,1);
        return res;
    }
    
    void helper(vector<vector<int>> &res,vector<int> &tmp,int n,int k,int num){
        if(k==0){
            res.push_back(tmp);
            return;
        }
        for(int i=num;i<=n-k+1;++i){
            tmp.push_back(i);
            helper(res,tmp,n,k-1,i+1);
            tmp.pop_back();
        }
    }
};