题目描述：
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).
本题与 Three Sum Zero类似，不同点在于，之前是寻找a+b+c=0的组合，现在是找最接近target的a+b+c的值。
思路：
依然是two pointer的思路。
1.对数组排序
2.使用a表示当前第i轮查找，b=i+1，表示第i轮的第1个元素，b∈[i+1,n)，c=len-1，一开始指向最后一个元素。
3.当找到a+b+c=target，直接返回；如果a+b+c < target，b++；否则，c--。
4.每次取Math.Abs(a+b+c- target)与当前Math.Abs(min-target)的最小值。
5.最后返回min
实现代码：public class Solution {
public int ThreeSumClosest(int[] nums, int target)
{
if(nums == null || nums.Length < 3){
return nums.Sum();
}
var list = nums.OrderBy(x=>x).ToList();
var len = list.Count;
int? min = null;
for (var i = 0 ;i <= len - 3 ;i++)
{
var a = list[i];
var start = i+1;
var end = len-1;
while (start < end)
{
var b = list[start];
var c = list[end];
if(a+b+c == target){
min = target;
break;
}
else if (a+b+c > target){
end --;
}
else{
start ++;
}
// update min
if (min == null || Math.Abs(a+b+c - target) < Math.Abs(min.Value - target))
{
min = a+b+c;
}
}
}
return min.Value;
}
}