Decode Ways LeetCode solution in Python

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# Runtime: 48 ms (Top 59.57%) | Memory: 14.1 MB (Top 30.54%)
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class Solution:
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    def numDecodings(self, s: str) -> int:
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        if s[0] == "0" or "00" in s:
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            return 0
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        l = len(s)
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        if l == 1:
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            return 1
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        elif l == 2:
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            if s[1] == "0":
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                if s[0] == "1" or s[0] == "2":
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                    return 1
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                else:
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                    return 0
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            else:
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                if int(s) <= 26:
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                    return 2
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                else:
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                    return 1
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        dp = [1]
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        if s[1] == "0":
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            if s[0] == "1" or s[0] == "2":
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                dp.append(1)
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            else:
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                return 0
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        else:
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            if int(s[:2]) <= 26:
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                dp.append(2)
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            else:
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                dp.append(1)
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        for i in range(2, l):
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            num = 0
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            if s[i] == "0":
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                if s[i - 1] != "1" and s[i - 1] != "2":
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                    return 0
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                else:
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                    num = dp[i - 2]
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            elif s[i - 1] == "1" or (s[i - 1] == "2" and int(f"{s[i-1]}{s[i]}") <= 26):
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                num = dp[i - 1] + dp[i - 2]
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            else:
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                num = dp[i - 1]
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            dp.append(num)
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        return dp[l - 1]