题目：
    345.反转字符串中的元音字母

    思路：
    对撞指针

    1. class Solution:
    2.     def reverseVowels(self, s: str) -> str:
    3.         s = list(s)
    4.         vowel_list = ['a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U']
    5.         left = 0
    6.         right = len(s) - 1
    7.         while left < right:
    8.             if s[left] not in vowel_list:
    9.                 left += 1
    10.             if s[right] not in vowel_list:
    11.                 right -= 1
    12.             if s[left] in vowel_list and s[right] in vowel_list:
    13.                 s[left], s[right] = s[right], s[left] #交换
    14.                 left += 1
    15.                 right -= 1
    16.         return ''.join(s)