/*
* [15] 3Sum
*
* https://leetcode.com/problems/3sum
*
* Medium (21.56%)
* Total Accepted:
* Total Submissions:
* Testcase Example: '[-1,0,1,2,-1,-4]'
*
* 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.
*
*
* For example, given array S = [-1, 0, 1, 2, -1, -4],
*
* A solution set is:
* [
* [-1, 0, 1],
* [-1, -1, 2]
* ]
*/三个数之和
转换为2个数之和
1.Map法
查找只需要O(1)
2.双指针法
很通用的一种方法 当为两个数 ，在[…]中找到两个数 a + b = target 若[…]没有排好序，先排序 然后两个指针分别从两边向中间集中 { if : a + b < target begin++ if : a + b > target end— } 通过这样的方式扫描完全部就可以得到解空间