Leetcode Medium 1 3Sum Closest

可惜就弄到了168 ms   64%：

思路：先排序。i, j, k 分别代表相加的三个数。

           i 开始循环， j 和 k 分别从 i+1 的位置和nums尾部互相靠拢（如果大于target：k-- ; 否则，j++）

class Solution:
    def threeSumClosest(self, nums, target):
        nums.sort()
        min_close = None
        for i in range(len(nums)):
            j = i + 1
            k = len(nums) - 1
            while j < k:
                sum_num = nums[i] + nums[j] + nums[k]
                
                if min_close == None or abs(sum_num - target) < abs(min_close - target):
                    min_close = sum_num
                    
                if sum_num == target:
                    return sum_num
                elif sum_num < target:
                    j += 1
                else:
                    k -= 1        
        return min_close