Find Common Characters LeetCode solution in Python

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# Runtime: 87 ms (Top 42.12%) | Memory: 14.3 MB (Top 6.17%)
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class Solution:
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    def _get_char_counts(self, s: str) -> dict[str]:
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        """builds a dict of letters : count"""
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        d = {}
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        for i in s:
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            d[i] = d.get(i, 0) + 1
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        return d
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    def commonChars(self, words: list[str]) -> list[str]:
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        """returns a string of letters common between a list of words (including duplicates)"""
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        if not words:
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            return
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        # O(n^2)
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        words = [self._get_char_counts(word) for word in words]
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        # O(nm), set intersection
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        common = words[0].keys()
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        for other in words[1:]:
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            common &= other.keys()
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        # O(nm), number of common characters across the number of words
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        result = []
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        for c in common:
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            result += [c] * min(count[c] for count in words)
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        return result