Leetcode: Combination Sum III

Question

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:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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Solution

class Solution(object):
    def combinationSum3(self, k, n):
        """ :type k: int :type n: int :rtype: List[List[int]] """

        res =[]
        if k>n or k==0 or n<0:
            return res

        self.helper(range(1,10), 0, k, n, [], res)
        return res

    def helper(self, lst, start, k, n, item, res):
        if k<0 or n<0:
            return 

        if k==0 and n==0:
            res.append(item)

        for ind in range(start,len(lst)):
            self.helper(lst, ind+1, k-1, n-lst[ind], item+[lst[ind]], res)