classic interview question done two ways in Swift

ReverseWordsInAString.swift

//
//  main.swift
//  ReverseWordsInAString
//
//  Created by Paul Wood on 8/30/19.
//  Copyright © 2019 Paul Wood. All rights reserved.
//

import Foundation

let sentence = "Words to reverse in a String"
let emptySentence = ""
let single = "a"

// Not very performant but very readable
func reverseWords(_ input: String) -> String {
  return input
    .components(separatedBy: .whitespacesAndNewlines)
    .map { (string) -> String in
      return String(string.reversed())
  }.joined(separator: " ")
}

// Reverse the words In place and in O(N) String is not subscriptable so we work around it with a [Character]
func reverseSentence(_ input: inout [Character]) {
  var wordStart = 0
  var wordEnd = 0
  var whitespace = 0
  
  func reverse(_ input: inout [Character], start: inout Int, end: inout Int) {
    while start < end {
      let temp = input[start]
      input[start] = input[end]
      input[end] = temp
      start += 1
      end -= 1
    }
  }
  
  while whitespace < input.count {
    if input[whitespace].isWhitespace {
      reverse(&input, start: &wordStart, end: &wordEnd)
      wordStart = whitespace + 1
    }
    whitespace += 1
    wordEnd = whitespace - 1
  }
  if whitespace != 0 {
    reverse(&input, start: &wordStart, end: &wordEnd)
  }
}

print(sentence)
print(reverseWords(sentence))
print(reverseWords(emptySentence))
print(reverseWords(single))

var sentenceArray = sentence.map { (c) -> Character in
    return c
}
reverseSentence(&sentenceArray)
print( String(sentenceArray))

var singleArray = single.map { (c) -> Character in
    return c
}
reverseSentence(&singleArray)
print( String(singleArray))

var emptyArray: [Character] = []
reverseSentence(&emptyArray)
print( String(emptyArray))