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| 标题： | 4Sum |
| 通过率： | 21.4% |
| 难度： | 中等 |
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
For example, given array S = {1 0 -1 0 -2 2}, and target = 0.
A solution set is:
(-1, 0, 0, 1)
(-2, -1, 1, 2)
(-2, 0, 0, 2)
4sum相当于在3sum外加一层循环：直接看代码：
1 class Solution: 2 # @return a list of lists of length 4, [[val1,val2,val3,val4]] 3 def fourSum(self, num, target): 4 num.sort() 5 ans = [] 6 for j in range(len(num)): 7 for i in range(j+1, len(num)): 8 l, r = i + 1, len(num) - 1 9 while l < r: 10 sum = num[j] + num[i] + num[l] + num[r] 11 if sum == target: 12 tmp=[num[j],num[i], num[l], num[r]] 13 if tmp not in ans: 14 ans.append(tmp) 15 l, r = l + 1, r - 1 16 elif sum < target: 17 l = l + 1 18 else: 19 r = r - 1 20 return ans
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原文地址：http://www.cnblogs.com/pkuYang/p/4403421.html
