Find Smallest Letter Greater Than Target LeetCode solution in Python

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class Solution:
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    def nextGreatestLetter(self, letters: List[str], target: str) -> str:
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        beg = 0
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        end = len(letters) - 1
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        while beg <= end:
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            mid = (beg + end) // 2
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            if letters[mid] > target:
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                end = mid - 1
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            else:
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                beg = mid + 1
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        return letters[beg] if beg < len(letters) else letters[0]