题目：
leetcode16题目描述
解法：
- 先对数组从小到大排序。
- 最外层遍历整个数组，再设置两个双指针，当三数之和sum大于target时，右指针向左移动一位；当三数之和sum小于target时，左指针向右移动一位。
- 设置一个diff是三数之和和target的差的绝对值，如果diff比当前的mindiff小，则替换mindiff。
具体代码如下：
class Solution:
def threeSumClosest(self, nums: List[int], target: int) -> int:
nums.sort()
res = 0
mindiff = 100000
for i in range(len(nums)):
left = i+1
right = len(nums)-1
while left<right:
sum = nums[left]+nums[right]+nums[i]
diff = abs(sum - target)
if diff < mindiff:
mindiff = diff
res = sum
if sum == target:
return sum
elif sum>target:
right -= 1
else:
left += 1
return res
