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

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

class Solution {
public:
    
    void combinedfs(int start,int end,int it,vector<int> &vec,vector<vector<int>>  &result)
    {
        if(it==0)
        {
            
            result.push_back(vec);
            return ;
        }
        
        for(int i=start;i<=end;i++)
        {
            
            
            vec.push_back(i);
            combinedfs(i+1,end,it-1,vec,result);
            vec.pop_back();
        }

        
    }
    
    vector<vector<int>> combine(int n, int k) {
     
        vector<int> vec;
        vector<vector<int>> result;
        
        
        combinedfs(1,n,k,vec,result);
        return result;

    }
};