Determine if String Halves Are Alike LeetCode solution in Python

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vowels = "aeiouAEIOU"
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class Solution:
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    def halvesAreAlike(self, S: str) -> bool:
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        mid, ans = len(S) // 2, 0
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        for i in range(mid):
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            if S[i] in vowels:
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                ans += 1
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            if S[mid + i] in vowels:
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                ans -= 1
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        return ans == 0