固定左邊數值，左右指針網中間趕

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:

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        sort(nums.begin(), nums.end());//将nums裡面的元素從小到大排序
    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;
    }
};