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],
]

不知道有没有更好的方法。

class Solution {
public:
    vector<vector<int> > re;
    vector<vector<int> > combine(int n, int k) {
        vector<int> m;
        if(n < k)return re;
        cal(n,0,k,m);
        return re;
    }
    void cal(int n, int begin,int num,vector<int> already)
    {
        if(begin != 0)
        already.push_back(begin);
        
        if(num == 0){
            re.push_back(already);
            return;
        }

        for(int i = begin + 1 ;i <= n - num+1  ;i++)
        {
            cal(n ,i , num-1,already);
        }
    }
};