16. 3Sum Closest

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).

和3sum的思想类似，先选一个元素a出来然后双指针指示b和c从两端扫描，这次是记录和target的最接近的值。
public static int threeSumClosest(int[] nums, int target)
{
Arrays.sort(nums);
int ret = 0;
int closest = Integer.MAX_VALUE;
int len = nums.length;
for (int i = 0; i < len - 2; i++)
{
if (i > 0 && nums[i] == nums[i - 1])
continue;
int l = i + 1;
int r = len - 1;
while (l < r)
{
int sum = nums[i] + nums[l] + nums[r];
if (sum < target)
{
if (target - sum < closest)
{
closest = target - sum;
ret = sum;
}
l++;
} else if (sum > target)
{
if (sum - target < closest)
{
closest = sum - target;
ret = sum;
}
r--;
} else
{
return sum;
}
}

}
return ret;
}