[LeetCode 16] 3Sum Closest

Title:

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).

int threeSumCloest(vector<int> &nums, int target){
	int result = 0;
	int min_gap = INT_MAX;
	
	sort(nums.begin(), nums.end());
	for (auto a = nums.begin(); a != prev(nums.end(), 2); a++)
	{
		auto b = next(a); //后继
		auto c = prev(nums.end());//前驱
		while (b < c)
		{	
			const int sum = *b + *c + *a;
			const int gap = abs(target - sum); //abs决定了这个值是最靠近target的
			if (gap < min_gap) //有更精确的值，刷新
			{
				result = sum;
				min_gap = gap;
			}
			if (sum<target)
				b++;
			else c--;
		}
		 
	}
 
	return result; 
}