Given array nums = [-1, 2, 1, -4], and target = 1.

The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

N sum系列。寻找与目标最相近的三数之和。
因为不用处理重复的问题，所以这道题反而更简单点

要注意的是在求和的值与目标相等时，就可以直接返回了

 1 class Solution:
 2     def threeSumClosest(self, nums, target):
 3         """
 4         :type nums: List[int]
 5         :type target: int
 6         :rtype: int
 7         """
 8         nums.sort()
 9         result = None
10         for i in range(len(nums)-2):
11             if i > 0 and nums[i] == nums[i-1]:
12                 continue
13             left, right = i + 1, len(nums) - 1
14             while left < right :
15                 s = nums[i] + nums[left] + nums[right]
16                 if result is None or abs(s - target) < abs(result - target) :
17                     result = s
18                 if s < target :
19                     left += 1
20                 elif s > target :
21                     right -= 1
22                 else :
23                     return target
24         return result