3Sum Closest

   找到与target最接近的三个数的和。

int three_sum_closest(vector<int> &num, int target) 

{

    int result = 0;

    int dist = INT_MAX;

    sort(num.begin(), num.end());

    for (vector<int>::const_iterator it = num.begin();

        it != num.end();

        ++it)

    {

        vector<int>::const_iterator front = it + 1;

        vector<int>::const_iterator back = num.end() - 1;

        while (front < back)

        {

            const int sum = *it + *front + *back;

            if (sum < target)

            {

                if (target - sum < dist)

                {

                    dist = target - sum;

                    result = sum;

                }

                ++front;

            }

            else if (sum > target)

            {

                if (sum - target < dist)

                {

                    dist = sum - target;

                    result = sum;

                }

                --back;

            }

            else

            {

                dist = 0;

                result = target;

                break;

            }

        }

    }

    return result;

}