leetcode 216. Combination Sum III

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.

Note:

• All numbers will be positive integers.
• The solution set must not contain duplicate combinations.

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]]

思路：回溯法

 1 class Solution {
 2 public:
 3     vectorint>> combinationSum3(int k, int n) {
 4         if (k <= 0)
 5             return {};
 6         vectorint> > res;
 7         vector<int> v;
 8         combinationSum3(k, n, res, v, 1);
 9         return res;
10     }
11 private:
12     void combinationSum3(int k, int n, vectorint> > &res, vector<int> &v, int begin) {
13         if (int(v.size()) == k && n == 0) {
14             res.push_back(v);
15             return ;
16         }
17         for (int i = begin; i <= 9 && n >= i; i++) {
18             v.push_back(i);
19             combinationSum3(k, n - i, res, v, i + 1);
20             v.pop_back();
21         }
22     }
23 };