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()