Next Greater Element III LeetCode solution in Python

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class Solution:
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    def nextGreaterElement(self, n):
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        digits = list(str(n))
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        i = len(digits) - 1
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        while i - 1 >= 0 and digits[i] <= digits[i - 1]:
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            i -= 1
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        if i == 0:
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            return -1
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        j = i
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        while j + 1 < len(digits) and digits[j + 1] > digits[i - 1]:
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            j += 1
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        digits[i - 1], digits[j] = digits[j], digits[i - 1]
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        digits[i:] = digits[i:][::-1]
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        ret = int("".join(digits))
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        return ret if ret < 1 << 31 else -1