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]]



与上面的题不同，此题的候选集变成1，2,3,4,5,6,7，8，9。另外对output的长度有限制。所以在搜索的时候我们要记录当前长度
当output的长度不符合要求的时候，及时return

class Solution {
public:
    vector<vector<int>> finalres ;
    vector<vector<int>> combinationSum3(int k, int target) {
        vector<int> curres;
        help(0,curres,1,k,target);
        return finalres;
        
    }
    void help(int cursum,vector<int>& curres,int index,int k,int target){
        if(cursum==target&& k==0)
            finalres.push_back(curres);
        if(cursum>target||k<0)
            return;
        for(int i = index;i<=9;i++){
            curres.push_back(i);
            help(cursum,curres,i+1,k-1,target-i);
            curres.pop_back();

        }
    }
};