Additive Number LeetCode solution in Python

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class Solution:
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    def isAdditiveNumber(self, num: str) -> bool:
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        def isadditive(num1, num2, st):
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            if len(st) == 0:
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                return True
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            num3 = str(num1 + num2)
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            l = len(num3)
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            return num3 == st[:l] and isadditive(num2, int(num3), st[l:])
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        for i in range(1, len(num) - 1):
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            for j in range(i + 1, len(num)):
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                if num[0] == "0" and i != 1:
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                    break
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                if num[i] == "0" and i + 1 != j:
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                    break
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                if isadditive(int(num[:i]), int(num[i:j]), num[j:]):
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                    return True
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        return False