题目描述：

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],
]

代码：

void get_combination(int n,int k,int start,vector<vector<int> > &combination,vector<int> vec)
{
    if(k == 0)
    {
        combination.push_back(vec);
        return;
    }
    else
    {
        for(int i = start;i <= n;i++)
        {
            vector<int> temp = vec;
            temp.push_back(i);
            get_combination(n,k-1,start+1,combination,temp);
        }
    }
}

vector<vector<int> > Solution::combine(int n, int k)
{
    vector<vector<int> > combination;
    vector<int> vec;
    get_combination(n,k,1,combination,vec);
    return combination;
}

LeetCode:Combinations