iOSCodeChallenge-reverse-vowels.swift

import UIKit
import XCTest

/*
 Thoughts/Assumptions:
 - If we are going to try to find a solution with a time complexity of O(n), we are going to have to get creative.
 - We could possibly use two converging indicies, one starting from the from of the string, the other starting from the end.
 - Because were taking in a String and returning a String, we could make an extension on String.
 - I should consider whether or not 'y' is included as a vowel.
 
 Test cases:
 "A test string".reverseVowels()    // "I test strang"
 "Hello".reverseVowels()            // "Holle"
 "She is Happy".reverseVowels()     // "Sha is Heppy"
 "She is Happy".reverseVowels()     // "Shy as Hippe"
 "Katie".reverseVowels()            // "Ketia"
 "N/A".reverseVowels()              // "N/A"
 "".reverseVowels()                 // ""
 
 Approach: Write a String extension, make a static variable of an array of vowels, and static var of an array of vowels with Y. Create a function with a boolean flag for whether not you want to use Y as a vowel. Create two indexes, one at the from of the array, and one at the back. With the first index, start looking at whether or not each index position is a vowel or not. If not, move to the next. If it is a vowel, find the first vowel starting from the back of the list, in the same way we found the vowel from the front. As long as the firstIndex < secondIndex, if second finds a vowel, swap it with the firstIndex vowel. Then start over from the first index vowel.
 */

extension String {
    
    static let vowels: Set<Character> = ["a","e","i","o","u","A","E","I","O","U"]
    static let vowelsY: Set<Character> = String.vowels.union(["y", "Y"])
    
    func reverseVowels(_ yIsAVowel: Bool = false) -> String {
        
        if self == "" { return "" }
        
        var chars = Array(self)
        var lhIndex = 0
        var rhIndex = chars.count - 1
        
        let vowelsToUse = yIsAVowel ? String.vowelsY : String.vowels
        
        while lhIndex < rhIndex {
            if !vowelsToUse.contains(chars[lhIndex]) {
                lhIndex += 1
            } else {
                if !vowelsToUse.contains(chars[rhIndex]) {
                    rhIndex -= 1
                } else {
                    chars.swapAt(lhIndex, rhIndex)
                    lhIndex += 1
                    rhIndex -= 1
                }
            }
        }
        return String(chars)
    }
}

// Test Cases
XCTAssert("A test string".reverseVowels() == "i test strAng")
XCTAssert("Hello".reverseVowels() == "Holle")
XCTAssert("She is Happy".reverseVowels() == "Sha is Heppy")
XCTAssert("She is Happy".reverseVowels(true) == "Shy as Hippe")
XCTAssert("Katie".reverseVowels() == "Ketia")
XCTAssert("N/A".reverseVowels() == "N/A")
XCTAssert("".reverseVowels() == "")