3Sum Closest

Given an array S of n integers, find three integers inS 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).就当作求线段之间的距离，先排序，通过三层递归，发现三个元素的和在一轮循环里成递增。