leetcode-216-组合总和③

题目描述：

方法一：回溯

class Solution:
    def combinationSum3(self, k: int, n: int) -> List[List[int]]:
        res = []
        def helper(k,n,start,tmp):
            if k==0:
                if n==0:
                    res.append(tmp)
                return
            for i in range(start,10):
                if n - i < 0:break
                helper(k-1,n-i,i+1,tmp+[i])
        helper(k,n,1,[])
        return res