leetcode（找出三个数数组和接近所给数）

题目：
   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.
   For example, given array S = {-1 2 1 -4}, and target = 1.
   The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).