LeetCode 1002: Find Common Characters

Meta Data

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Difficulty: easy First Attempt: 2022-09-27

• Submissions: Runtime Error → Accepted ×3

Approach

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Each character appears in the answer as many times as its minimum count over all words (Hash Table Template 1). Python’s Counter & Counter takes per-character minimums; intersect over all words and expand with elements().

My Attempts

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Attempt 1 intersected a set with a string (set & str, TypeError), and a set drops multiplicity anyway: ["bb"] should give two bs. Switching to Counter & passed.

Solution

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class Solution:
    def commonChars(self, words: List[str]) -> List[str]:
        common = Counter(words[0])
        for w in words[1:]:
            common &= Counter(w)
        return list(common.elements())

Complexity

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Time O(total characters), space O(k).

Mistakes to Avoid

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• set loses multiplicity: ["bb"] gives ["b", "b"].
• set & str mixes types and raises TypeError.

Interview Explanation

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I count each word and intersect the Counters, taking each character’s minimum count, then expand into a list. Time is linear in the total length.