3 Sum 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.

Example:

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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).

思路：
思路和3sum类似，还是找一个基准值和两个指针lp和rp不过有些地方简化了，只需要获得一个值。因此，对应的组合不用存取，只需要不断的更新最接近的值即可。

实现：

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def threeSumClosest(nums, target):
        if not len(nums):  
            return 0

        nums.sort()
        res = nums[0] + nums[1] + nums[2]   
        for i in range(len(nums) - 2):  
            lp = i + 1  
            rp = len(nums) - 1  
            while lp < rp:  
                closest = nums[i] + nums[lp] + nums[rp]  
                if abs(closest - target) < abs(res - target):  
                    res = closest  
                if closest < target:  
                    lp += 1  
                elif closest > target:  
                    rp -= 1  
                else:  
                    lp += 1  
                    rp -= 1  
        return res