Check If String Is Transformable With Substring Sort Operations LeetCode solution in Python

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from collections import defaultdict
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class Solution:
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    def isTransformable(self, s: str, t: str) -> bool:
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        index = defaultdict(list)
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        for i, c in enumerate(s):
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            index[c].append(i)
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        curpos = defaultdict(int)
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        for c in t:
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            if curpos[c] == len(index[c]):
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                return False
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            for i in range(int(c)):
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                i = str(i)
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                if (
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                    curpos[i] < len(index[i])
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                    and index[i][curpos[i]] < index[c][curpos[c]]
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                ):
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                    return False
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            curpos[c] += 1
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        return True