LeetCode 345: Reverse Vowels of a String

Meta Data

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Difficulty: easy First Attempt: 2022-08-23

• Submissions: Accepted ×3

Approach

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Two pointers each skip non-vowels and swap when both point at vowels (Two Pointers Template 1). Vowels include uppercase, so test against the set "aeiouAEIOU".

My Solution

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class Solution:
    def reverseVowels(self, s: str) -> str:
        vowels = set("aeiouAEIOU")
        a = list(s)
        l, r = 0, len(a) - 1
        while l < r:
            if a[l] not in vowels:
                l += 1
            elif a[r] not in vowels:
                r -= 1
            else:
                a[l], a[r] = a[r], a[l]
                l, r = l + 1, r - 1
        return "".join(a)

Complexity

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

Mistakes to Avoid

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• Uppercase vowels count too.

Interview Explanation

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Two pointers skip non-vowels and swap when both are vowels, then move inward. Time O(n).