Largest Number After Mutating Substring LeetCode solution in Python

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class Solution:
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    def maximumNumber(self, num: str, change: List[int]) -> str:
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        flag = 0
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        ls = list(num)
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        for i in range(len(ls)):
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            k = int(ls[i])
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            if change[k] > k:
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                ls[i] = str(change[k])
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                flag = 1
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            elif flag == 1 and change[k] < k:
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                break
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        return "".join(ls)