Leetcode #345: Reverse Vowels of a String

func reverseVowels(_ s: String) -> String {
    let vowel: Set<Character> = ["a", "e", "i", "o", "u", "A", "E", "I", "O", "U"]
    var start = 0
    var end = s.count - 1
    var str = Array(s)
    while start < end {
        let isStartCharVowel = vowel.contains(str[start])
        let isEndCharVowel = vowel.contains(str[end])
        if isStartCharVowel && isEndCharVowel {
            str.swapAt(start, end)
            start += 1
            end -= 1
        } else if isStartCharVowel {
            end -= 1
        } else if isEndCharVowel {
            start += 1
        } else {
            start += 1
            end -= 1
        }
    }
    return String(str)
}