有了前面的基础这道题的思路也很清晰了，无非是把4Sum转换为3Sum解决。

这里实现Nsum完成对此类题的通用解法。

 1 class Solution:
 2     def twoSum(self, nums, target, index):
 3         left, right = index, len(nums) - 1
 4         result = []
 5         while left < right:
 6             sum = nums[left] + nums[right]
 7             if sum < target:
 8                 left += 1
 9             elif sum > target:
10                 right -= 1
11             else:
12                 result.append([nums[left], nums[right]])
13                 while left < right and nums[left] == nums[left + 1]:
14                     left += 1
15                 while left < right and nums[right] == nums[right - 1]:
16                     right -= 1
17                 left += 1
18                 right -= 1
19         return result
20     def nSum(self, nums, target, index, n):
21         result = []
22         if n == 2:
23             return self.twoSum(nums, target, index)
24         else:
25             i = index
26             while i < len(nums) - n + 1 :
27                 if i > index and nums[i] == nums[i-1]:
28                     i += 1
29                     continue
30                 temp = self.nSum(nums, target - nums[i], i + 1, n - 1)
31                 if len(temp) > 0 :
32                     for t in temp:
33                         t.append(nums[i])
34                         result.append(t)
35                 i += 1
36         return result
37     def fourSum(self, nums, target):
38         nums.sort()
39         return self.nSum(nums, target, 0, 4)