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]]



與上面的題不一樣，此題的候選集變成1，2,3,4,5,6,7，8，9。另外對output的長度有限制。因此在搜索的時候咱們要記錄當前長度
當output的長度不符合要求的時候，及時return

 1 class Solution {
 2 public:
 3     vector<vector<int>> finalres ;
 4     vector<vector<int>> combinationSum3(int k, int target) {
 5         vector<int> curres;
 6         help(0,curres,1,k,target);
 7         return finalres;
 8         
 9     }
10     void help(int cursum,vector<int>& curres,int index,int k,int target){
11         if(cursum==target&& k==0)
12             finalres.push_back(curres);
13         if(cursum>target||k<0)
14             return;
15         for(int i = index;i<=9;i++){
16             curres.push_back(i);
17             help(cursum,curres,i+1,k-1,target-i);
18             curres.pop_back();
19 
20         }
21     }
22 };