经典算法面试题系列 （二）——three sum

接着two sum，three sum的难度上升了不少。因为two sum的答案是唯一的，three sum的答案是多个。题目链接https://leetcode.com/problems/3sum/#/description。

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 duplicat