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).

題目大意：給定一個長度為n的整型陣列和一個整數target，在陣列中找出三個數，它們的和最接近target。可以認為對於每個測試用例，解決方案都是唯一的。

解題思路：類似《15. 3Sum（求陣列中和為0的3個數）》，對於要求三個數的和，可以先對陣列從小到大排序，然後鎖定一個數nums[i]，根據雙指標的思路從陣列兩端到中間遍歷，找到兩個數nums[left]和nums[right]，使得nums[left]+nums[right]+nums[i]的和最接近target。遍歷過程中使用兩個整數diff和res記錄當前獲得的最優解，其中res表示到目前為止找到的最接近target的三個數的和，diff表示target與res的差的絕對值。當遍歷到res==target，已經是最佳方案了，直接返回。

解題程式碼：（22ms，beats 65.39%）

class Solution {
    public int threeSumClosest(int[] nums, int target) {
        int diff = Integer.MAX_VALUE;
		int res = 0;
		int len = nums.length;
		Arrays.sort(nums);
		for (int i = 0; i < len; i++) {
			int left = i + 1;
			int right = len - 1;
			while (left < right) {
				int sum = nums[i] + nums[left] + nums[right];
				if (sum == target) {
					return sum;
				}
				if (Math.abs(sum - target) < diff) {
					diff = Math.abs(sum - target);
					res = sum;
				}
				if (sum > target) {
					right--;
				} else {
					left++;
				}
			}
		}
		return res;
    }
}