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.

 

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]]

这一题和combiantion sum I/II 其实很类似。只不过candidates只有[1,2,...,9]。而且只有当len(line) == k 并且 sum(line) = n才把line添加到res里面。

class Solution(object):
    def combinationSum3(self, k, n):
        """
        :type k: int
        :type n: int
        :rtype: List[List[int]]
        """
        nums = list(range(1,10))
        res = []
        self.helper(nums, k, n, res, [])
        return res
        
    def helper(self, nums, k, target, res, line):
        if target == 0 and len(line) == k:
            res.append([x for x in line])
        
        for i, x in enumerate(nums):
            if x <= target:
                line.append(x)
                self.helper(nums[i+1:], k, target -x, res, line)
                line.pop()