题目描述如下：
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], ]
一个简单的递归就可以了：
class Solution {
public:
int n;
int k;
vector<vector<int> > combine(int tn,int k)
{
n=tn;
vector<vector<int> > ans;
vector<int> tmp;
_combineAll(0,k,ans,tmp);
return ans;
}
void _combineAll(int cur,int need,vector<vector<int> >& ans,vector<int>& tmp)
{
if (need==0)
{
ans.push_back(tmp);
return;
}
if (n-cur<need)
return;
tmp.push_back(cur+1);
_combineAll(cur+1,need-1,ans,tmp);
tmp.pop_back();
_combineAll(cur+1,need,ans,tmp);
}
};