Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Ensure that numbers within the set are sorted in ascending order.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

Credits:

基本思路：

使用dfs进行穷举遍历。

因为结果集要求按升序排列，则先选择较小的数字，然后从剩下的较大数中进行选择。

确保每次选择的数都大于上次选择的。

class Solution {public:    vector
     
      
       > combinationSum3(int k, int n) {        vector
       
        
         > ans;        vector
         
           seq;        helper(ans, seq, k, n);        return ans;    }        void helper(vector
          
           
            >& ans, vector
            
             & seq, int k, int n) { if (!k && !n) return ans.push_back(seq); if (!k || n<0) return; int start = seq.empty() ? 1 : seq.back()+1; for (int i=start; i<=9; i++) { seq.push_back(i); helper(ans, seq, k-1, n-i); seq.pop_back(); } }};