LC824 Goat Latin

class Solution:
# time O(N+2K^2)   space(N)
    def toGoatLatin(self, S: str) -> str:
        if not S:
            return []
        sen = S.split(' ') # O(N)
        vo  = {'a','e','i','o','u'}
        res = list()
        for idx, word in enumerate(sen):
            if word[0].lower() not in vo:  # edge case: Capical vowel 
                word = word[1:] + word[0]  # O(w) w is word 
            word = word+'ma' + 'a'*(idx+1)  # O(K^2) in total loop  
            res.append(word) # can't use res[idx] = word. will cause out of bound error
        return ' '.join(res)