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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，实现代码：
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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
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