15 3Sum（medium）
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.
给定n个整数的数组S，S中有元素a，b，c，使得+ b + c = 0？ 查找数组中所有独特的三元组，给出的和为零。
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.
给定n个整数的数组S，S中有元素a，b，c，使得+ b + c = 0？ 查找数组中所有独特的三元组，给出的和为零。
