3Sum-Leetcode

Given an array S ofn integers, are there elementsa, 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.

For example, given array S = [-1, 0, 1, 2, -1, -4],A solution set is:[  [-1, 0, 1],  [-1, -1, 2]]从头开始遍历，i为数组第一个的值，pre为数组第二个的值，back为数组的第三个的值，主要是以向中靠拢的原则。还有就是由于不是每个元素是独一的，要遍历相同的元素。