Given an array ​​nums​​ of n integers and an integer ​​target​​​, find three integers in ​​nums​​​ such that the sum is closest to ​​target​​. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example:

Given array nums = [-1, 2, 1, -4], and target = 1. The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());//将nums里面的元素从小到大排序
    int preSum,left,right,sum;
    bool First = true;
    for (int i = 0; i < nums.size(); i++)
    {
        left = i + 1; right = nums.size() - 1;
        while (left < right)
        {
            sum = nums[i] + nums[left] + nums[right];
            if (First) {//第一次将preSum初始化
                preSum = sum;
                First = false;
            }
            else {
                if (abs(target - sum) < abs(target - preSum)) {
                    preSum = sum;
                }
            }
            if (preSum == target) { return preSum; }
            if (sum > target) { right--; }
            else { left++; }
        }
    }
    return preSum;
    }
};