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).

AC代码：

class Solution(object):
    def threeSumClosest(self, nums, target):
        if len(nums) < 3: return 0
        ret_num = nums[0] + nums[1] + nums[2]
        nums.sort()
        for i_1st in xrange(len(nums) - 2):
            # remove duplicates
            if i_1st > 0 and nums[i_1st] == nums[i_1st - 1]: continue
            i_2nd, i_3rd = i_1st + 1, len(nums) - 1
            while i_2nd < i_3rd:
                s = nums[i_1st] + nums[i_2nd] + nums[i_3rd]
                if s == target:
                    return target
                if s > target:
                    i_3rd -= 1
                else:
                    i_2nd += 1
                if abs(s - target) < abs(ret_num - target):
                    ret_num = s
        return ret_num

本题和15. 3Sum的双指针解法基本一模一样，只不过本题求的是最接近目标值的和，思想上是一样的。