Reverse vowels of a string

Reverse only the vowels and leave every other character in place. Two pointers walk in from both ends, skip consonants, and swap whenever both rest on a vowel.

The idea

Reversing the vowels means the first vowel swaps with the last vowel, the second with the second to last, and so on. Two pointers find those pairs directly: l moves right until it sits on a vowel, r moves left until it does too, then they swap and both step inward. Consonants are never touched.

Code

public string ReverseVowels(string s) {
    var c = s.ToCharArray();
    int l = 0, r = c.Length - 1;
    while (l < r) {
        if (!IsVowel(c[l])) l++; // not a vowel: it stays put
        else if (!IsVowel(c[r])) r--;
        else { (c[l], c[r]) = (c[r], c[l]); l++; r--; } // both vowels: swap
    }
    return new string(c);
}

static bool IsVowel(char ch) => "aeiouAEIOU".IndexOf(ch) >= 0;

Tips

• Uppercase vowels count too: "IceCreAm" becomes "AceCreIm".
• A string with an odd number of vowels leaves the middle vowel where it is, because l and r meet on it.
• Strings are immutable in C#, so the work happens on a char array that is turned back into a string at the end.

In the .NET library

string.ToCharArray() copies the characters into a fresh array; new string(char[]) copies them back. For long inputs, string.Create or a Span<char> avoids one of the copies.