[easy][String][Two-pointers]345.Reverse vowels

原题是：

Write a function that takes a string as input and reverse only the vowels of a string.

Example 1:
Given s = "hello", return "holle".

Example 2:
Given s = "leetcode", return "leotcede".

Note:
The vowels does not include the letter "y".

思路是：

两个指针分别从头尾去寻找元音，两个找到的时候，就交换。一个是，另一个不是的时候，挪动不是元音的那个指针。

代码是：

class Solution:
    def reverseVowels(self, s):
        """
        :type s: str
        :rtype: str
        """
        
        i = 0 
        j = len(s) - 1
        length = len(s)
        strlist = list(s)
        vowels = set(['a','e','i','o','u','A','E','I','O','U'])
        while(i < j):
            if strlist[i] in vowels and strlist[j] in vowels:
                tmp = strlist[i]
                strlist[i] = strlist[j]
                strlist[j] = tmp
                i += 1
                j -= 1
            elif strlist[i] in vowels and strlist[j] not in vowels:
                j -= 1
            else :
                i += 1
        return ''.join(strlist)