def reverse_vowels(s):
    vowels = set('aeiouyAEIOUY')
    s = list(s)
    left, right = 0, len(s) - 1
    while left < right:
        if s[left] in vowels and s[right] in vowels:
            s[left], s[right] = s[right], s[left]
            left += 1
            right -= 1
        elif s[left] in vowels:
            right -= 1
        elif s[right] in vowels:
            left += 1
        else:
            left += 1
            right -= 1
    return ''.join(s)