Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.
思路： 排序，然后two pointer,复杂度 O(n^2)
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阅读更多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: The solution set must not contain duplicate quadruplets.
思路： O(n^2)枚举两个元素，变成2-sum问题，总体复杂度O(n^3)
阅读更多Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
Note: The solution set must not contain duplicate triplets.
思路：排序O(nlgn)，然后枚举一个元素O(n),对于每个元素，在剩下的区间中 two pointer O(n)
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