Letter Case Permutation LeetCode solution in Python

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class Solution(object):
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    def letterCasePermutation(self, s):
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        if s == "":
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            return [""]
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        t = s[0].lower()
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        li = []
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        res = self.letterCasePermutation(s[1:])
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        for i in res:
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            li.append(t + i)
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        if t not in "1234567890":
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            for i in res:
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                li.append(t.upper() + i)
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        return li