LeetCode 3120: Count the Number of Special Characters I

Meta Data

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Difficulty: easy First Attempt: 2024-04-21

• Submissions: Accepted (first try)

Approach

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Split the seen letters into two sets, those seen in lowercase and those seen in uppercase (stored in one case), and count the letters present in both (Hash Table).

My Solution

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class Solution:
    def numberOfSpecialChars(self, word: str) -> int:
        lower = {c for c in word if c.islower()}
        upper = {c.lower() for c in word if c.isupper()}
        return len(lower & upper)

Complexity

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Time O(n), space O(26).

Mistakes to Avoid

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• Store both sets in the same case before intersecting.

Interview Explanation

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I put letters seen in lowercase and in uppercase into two sets in one case and return the size of their intersection. Time O(n).