A few ways to reverse a string in Swift

reverse-a-string-algorithm.swift

let word = "abcdefg"

//Using swift lib
String(word.characters.reverse()) //"gfedcba"

//Using char array and insert
func reverseString(wordToReverse: String) -> String {
    guard wordToReverse.characters.count > 1 else {
        return wordToReverse
    }

    var reversedWord = [Character]()
    
    for char in wordToReverse.characters {
        reversedWord.insert(char, atIndex: 0)
    }
    return String(reversedWord)
}
assert(reverseString(word) == "gfedcba")

//Swap in-place, using char array
func reverseString2(wordToReverse: String) -> String {
    guard wordToReverse.characters.count > 1 else {
        return wordToReverse
    }
    
    var newWord = Array(wordToReverse.characters)
    let maxIndex = newWord.count - 1
    
    for i in 0...maxIndex {
        if i > maxIndex - i {
            break
        }
        (newWord[i], newWord[maxIndex - i]) = (newWord[maxIndex - i], newWord[i])
    }
    return String(newWord)
}

assert(reverseString2(word) == "gfedcba")

@jmohanr here is what you look for: Time complexity: O(N), Space comp. : O(N)

func reverseWithSpaces(of str: String) -> String {
    var newStr = [Character]()
    
    var lastIndexOffset = str.count - 1
    for char in str.enumerated() {
        if char.element == " " {
            newStr.insert(" ", at: 0)
        } else {
            let index = str.index(str.startIndex, offsetBy: lastIndexOffset)
            newStr.insert(str[index], at: 0)
            lastIndexOffset -= 1
        }
    }
    
    return String(newStr)
}

// Test

let apple = "I am apple"
let reversedApple = reverseWithSpaces(of: apple)