問題描述：

Given an arraySofnintegers, find three integers inSsuch 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.

示例：

 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).

問題分析：

要求三個數的和最接近目標值，可分為以下幾個步驟：

1.將陣列升序排序

2.設定兩個指標 front 和 back 分別指向陣列的前段和後端

3.先選取好第一個元素，然後遍歷第二和第三個元素，將三者的和與目標值target作比較，選取距離較近的

過程詳見程式碼：

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        int res = 0;
        int dist = INT_MAX;
        sort(nums.begin(), nums.end());//排序 
        
        for(int i = 0; i < nums.size(); i++)
        {
        	int front = i + 1;
        	int back = nums.size() - 1;
        	
        	while(front < back)
        	{
        		int sum = nums[i] + nums[front] + nums[back];
        		if(sum < target) 
        		{
        			int temp = target - sum;
        			if(temp < dist)
        			{
        				dist = temp;
        				res = sum;
					}
        			front++;
				}
				else if(sum > target)
				{
					int temp = sum - target;
        			if(temp < dist)
        			{
        				dist = temp;
        				res = sum;
					}
					back--;
				}
				else
				{
					return target;
				}
			}
		}
		
		return res;
    }
};