Leetcode 39. Combination Sum

Posted 2020-08-26 lettuan

Given a set of candidate numbers (C) (without duplicates) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

For example, given candidate set [2, 3, 6, 7] and target 7,
A solution set is:

[
  [7],
  [2, 2, 3]
]

一般跟求combiantion相关的题目基本上都会用到DFS。这里需要注意的是，用L22可以代替L23-L25。因为L22并没有改变 line， 所以line+[x]在内存中其实被另外的单独存储，所以不用line.pop()。

 1 class Solution(object):
 2     def combinationSum(self, candidates, target):
 3         """
 4         :type candidates: List[int]
 5         :type target: int
 6         :rtype: List[List[int]]
 7         """
 8         candidates.sort()
 9         
10         res = []
11         line = []
12         self.helper(candidates, target, res, line)
13         return res
14         
15     def helper(self, candidates, target, res, line):
16         if target == 0:
17             res.append([x for x in line])
18             return 
19     
20         for i, x in enumerate(candidates):
21             if x <= target:
22             #    self.helper(candidates[i:], target-x, res, line+[x])
23                 line.append(x)
24                 self.helper(candidates[i:], target-x, res, line)
25                 line.pop()
26