I was given a simple requirement for reversing a string. The requirement is as follows:

In any programming language, create an input that accepts "My name is Albert McDonald." and outputs the reversed value "Ym eman si Trebla DlAnodcm."

I got it to work like this:

string expectedInput = "My name is Albert McDonald.";

//Prompt the user to enter "My name is Albert McDonald."
Console.WriteLine("Please enter the sentence \"" + expectedInput + "\"");

//Wait for the users input.
string inputText = Console.ReadLine();

//Convert the input to a char array.
List<char> characters = inputText.ToList<char>();

//Get the list of uppercase indexes
List<int> upperIndexes = new List<int>();
for (int index = 0; index < characters.Count; index++)
    if (Char.IsUpper(characters[index]))

//Remove the period at the end
inputText = inputText.Substring(0, inputText.Length - 1);

//Get the reversed array of words.
string[] words = inputText.Split(' ').Reverse().ToArray();

//Join the array back to a string.
string revesedWords = string.Join(" ", words);

//Reverse the entire string and make it all lower case.
string result = new string(revesedWords.Reverse().ToArray()).ToLower();
StringBuilder sb = new StringBuilder(result);

//Change the case of the original index
foreach (int index in upperIndexes)
    char newCharacter = char.Parse(result.Substring(index, 1).ToUpper());
    sb[index] = newCharacter;

//Output the resutlt
Console.WriteLine("Your name in reverse by word is: \"" + sb.ToString() + ".\"");

//Inform the user how to close the screen.
Console.WriteLine("Press any key to close the screen.");

Is there a better way in terms of lines of code and performance-wise?

  • 1
    \$\begingroup\$ So, each word is reversed separately? How exactly is a ”word” defined? For example, should 123 considered as a word? What about ?!? \$\endgroup\$
    – svick
    Commented Apr 22, 2013 at 14:38

2 Answers 2


If you aim for fewer lines of code, you could of course put everything into a single line, but that would not helpful at all.

However, you could shorten your code a bit by using a simple regular expression and some linq-fu, something like:

var s = yourInputString;

// split string into words
var splitted = Regex.Split(s, @"(\W)").Select(r => r.AsEnumerable()).ToArray();

// reverse each word
var reversed = splitted.Select(sp => sp.Reverse().ToArray());

// adjust case
var zipped = splitted.Zip(reversed, (a, b) => string.Join("", a.Select((c, i) => char.IsUpper(c) ? 
                                                                                 char.ToUpper(b[i]) : 

// join back to string
var result = string.Join("", zipped);

I would not care about performance in this case, because it would be pointless.

Also, reversing a string with Reverse().ToArray() will corrupt your string if it contains surrogate characters, but this is probably out of scope of your task.

  • \$\begingroup\$ Is the Where() necessary? I think it won't actually affect the output. \$\endgroup\$
    – svick
    Commented Apr 22, 2013 at 14:42

My algorithm is roughly the same. However it changes some things that you might find quite obvious after reading it.

    static string ReverseString(string str)
        string[] arr = str.Split(' ');
        string result = "";

        foreach (var s in arr)
            for (int i = s.Length - 1; i >= 0; i--)
                if (Char.IsUpper(s[s.Length - 1 - i]))
                    result += s[i].ToString().ToUpper();
                    result += s[i].ToString().ToLower();
            result += " ";

        return result;

Another thing that might be helpful is knowing whether punctuation symbols are part of the words or not. That's easy to catch though.

  • \$\begingroup\$ Could you explain what exactly did you change? \$\endgroup\$
    – svick
    Commented Apr 22, 2013 at 18:52
  • \$\begingroup\$ well for instance I don't use .Reverse() which saves me one method call (almost nothing) but more important allows me not to use an index list and saving me another loop, although is still O(n). \$\endgroup\$ Commented Apr 22, 2013 at 19:10

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