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 = 2
Output:
[
  [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<vector<int>> combine(int n, int k) {
        vector<vector<int>> results;
        vector<int> temp;
        backtracking(results, temp, n, k, 1);
        return results;
    }
    
    void backtracking(vector<vector<int>>& results, vector<int>& 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();
        }
    }