算法描述：

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2Output:[  [2,4],  [3,4],  [2,3],  [1,2],  [1,3],  [1,4],]

解题思路：题目要求给出所有肯能的组合，首先想到了回溯法。需要注意的是下一次迭代的起始是i+1，这样去除之前用过的数。

vector
     
      
       > combine(int n, int k) {        vector
       
        
         > results;        vector
         
           temp;        backtracking(results, temp, n, k, 1);        return results;    }        void backtracking(vector
          
           
            >& results, vector
            
             & temp, int n, int k, int index){ if(temp.size() == k){ results.push_back(temp); return; } for(int i=index; i <= n; i++){ temp.push_back(i); backtracking(results, temp, n , k, i+1); temp.pop_back(); } }