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🧩 Problem Statement

Given a string s, reverse only the vowels of the string and return the resulting string.

🔢 Example

python

Input: s = "hello"
Output: "holle"

Input: s = "leetcode"
Output: "leotcede"

✅ Constraints

• 1 <= s.length <= 3 * 10⁵
• s consists of printable ASCII characters.

💡 Key Concept: Two-Pointer Strategy

This problem is a classic case for the two-pointer approach where you:

• Start one pointer from the left (i)
• Start another from the right (j)
• Move inward until both pointers find vowels, then swap them
• Continue until the pointers meet

🧠 Step-by-Step Plan

1. Define a set of vowels: {'a', 'e', 'i', 'o', 'u'}
2. Use two pointers:

• i = 0 (start)
• j = len(s) - 1 (end)

1. Move i forward and j backward until both point to vowels
2. Swap the vowels
3. Join and return the modified string

✅ Python Code – Two-Pointer Approach

python

class Solution:
    def reverseVowels(self, s: str) -> str:
        vowels = set("aeiouAEIOU")
        s = list(s)  # Convert string to list for in-place swaps
        i, j = 0, len(s) - 1

        while i < j:
            if s[i] not in vowels:
                i += 1
                continue
            if s[j] not in vowels:
                j -= 1
                continue
            s[i], s[j] = s[j], s[i]
            i += 1
            j -= 1

        return ''.join(s)

🔍 Example Walkthrough: "hello"

• s = ["h", "e", "l", "l", "o"]
• Vowels: e (index 1), o (index 4)
• Swap → ["h", "o", "l", "l", "e"]
• Result = "holle"

⏱️ Time and Space Complexity

MetricValueTime ComplexityO(n)Space ComplexityO(n)

Note: Space is O(n) because strings are immutable in Python, so we convert it to a list for modification.

📦 Edge Test Cases

python

Input: "aA"         → Output: "Aa"
Input: "bcdfg"      → Output: "bcdfg"  # No vowels
Input: "Euston"     → Output: "oustEn"
Input: "AaEeIiOoUu" → Output: "uUoOiIeEaA"

✅ Conclusion

LeetCode 345: Reverse Vowels of a String is a great follow-up to the classic reverse string problem. It strengthens your grip on the two-pointer technique, string traversal, and character-level manipulation.

This problem often appears in interviews to test how well you can modify a subset of a structure in-place and efficiently.