Minimum Swaps to Make Strings Equal LeetCode solution in Python

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class Solution:
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    def minimumSwap(self, s1: str, s2: str) -> int:
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        h = defaultdict(int)
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        count = 0  # variable to keep track of the number of mismatches; it is impossible to make strings equal if count is odd
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        for i in range(len(s1)):
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            if s1[i] != s2[i]:
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                count += 1
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                h[s1[i]] += 1
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        if count % 2 != 0:
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            return -1
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        res, a, b = 0, h["x"], h["y"]
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        res += a // 2 + b // 2
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        if a % 2 == 0:
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            return res
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        return res + 2