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], ]回溯算法，實現代碼：
class Solution {
public:
vector > combine(int n, int k) {
vector> res;
if(n<1||k<1||n temp;
collect(n,k,0,temp,res);
return res;
}
void collect(int n,int k,int i,vector &temp,vector> &res)
{
if(temp.size()==k)
{
res.push_back(temp);
return;
}
for(;i
