Minimum Genetic Mutation LeetCode solution in Python

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class Solution:
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    def minMutation(self, start: str, end: str, bank: List[str]) -> int:
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        q = deque()
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        q.append(start)
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        n = len(bank)
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        last = 0
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        used = [False] * n
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        for i, x in enumerate(bank):
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            if start == x:
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                used[i] = True
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            if end == x:
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                last = i
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        dist = 0
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        while q:
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            dist += 1
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            for _ in range(len(q)):
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                w = q.popleft()
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                for i, x in enumerate(bank):
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                    if used[i]:
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                        continue
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                    bad = 0
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                    for j in range(8):
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                        if w[j] != x[j]:
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                            bad += 1
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                            if bad == 2:
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                                break
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                    if bad == 1:
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                        if last == i:
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                            return dist
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                        used[i] = True
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                        q.append(x)
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        return -1