3Sum Closest

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).

　　这道题原理和前面的3Sum一样，不再说明。

solution：

int threeSumClosest(vector<int> &num, int target) {
    int n = num.size();
    if(n <= 3)
        return accumulate(num.begin(),num.end(),0);
    sort(num.begin(),num.end());
    int res = num[0] + num[1] + num[2];
    int low, high;
    int sum;
    for (int i = 0;i < n-2;++i)
    {
        low = i + 1;
        high = n - 1;
        while (low < high)
        {
            sum = num[i] + num[low] + num[high];
            if (abs(sum - target) < abs(res - target))
            {
                if(sum == target)
                    return sum;
                res = sum;
            }
            sum < target ? ++low : --high; 
        }
    }
    return res;
}