class Solution {
    let vowels: Set<Character> = ["a", "e", "i", "o", "u", "A", "E", "I", "O", "U"]
    func reverseVowels(_ s: String) -> String {
        var foundIndices: [String.Index] = []
        var ans = s
        for i in s.indices {
            if vowels.contains(s[i]) {
                foundIndices.append(i)
            }
        }

        var l: String.Index?
        var r: String.Index?

        while let l = l, let r = r, l < r, l < s.endIndex, r >= s.startIndex {
            
            // HOW CAN I SWAP THE CHARACTERS AT DIFFERENT INDEXES?
            // ABANDONING because `String.Index` doesn't swap nor it allows you to set a character for a given index. I suppose it's because you can't guarantee that whatever value you're replacing has the same character size...
            s.formIndex(after:l)
            s.formIndex(before:r)
        }
        return ans
    }
}