using System;

using System.Linq;

​

Console.WriteLine(LongString("lol", 10));

Console.WriteLine(LongString("hello world", 3));

Console.WriteLine(LongString2("lol", 10));

Console.WriteLine(LongString2("hello world", 3));

​

string LongString(string s, int i) => string.Join("", s.Select(x => x switch

    {

        'a' or 'e' or 'i' or 'o' or 'u' => "".PadLeft(i, x),

        _ => $"{x}"

    }));

​

​

string LongString2(string s, int i) => string.Join("", s.Select(x => "aeiou".Contains(x) ? "".PadLeft(i, x) : $"{x}"));