using System;

using System.Text;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var test = "abc";

        Console.WriteLine(test.RecursiveReverse());

    }

}

​

public static class Helper

{

    public static string ReverseUsingForBackward(this string text)

    {

        string output = string.Empty;

        for (int i = text.Length - 1; i >= 0; i--)

        {

            output += text[i];

        }

​

        return output;

    }

    public static string ReverseUsingForForward(this string text)

    {

        string output = string.Empty;

        for (int i = 0; i <= text.Length - 1; i++)

        {

            output = text[i] + output;

        }

​

        return output;

    }

    public static string ReverseUsingForeach(this string input)

    {

        string output = string.Empty;

        foreach(var i in input)

            output = $"{i}{output}";

        return output;

    }

    public static string ReverseUsingBuiltInFunction(this string input)

    {

        char[] charArray = input.ToCharArray();

        Array.Reverse( charArray );

        return new string( charArray );

​

    }

    public static string ReverseXor(this string s)

    {

        if (s == null) return null;

        char[] charArray = s.ToCharArray();

        int len = s.Length - 1;

​

        for (int i = 0; i < len; i++, len--)

        {

            charArray[i] ^= charArray[len];

            charArray[len] ^= charArray[i];

            charArray[i] ^= charArray[len];

        }

​

        return new string(charArray);

    }

    public static string ReverseUsingStringBuilder(this string text)

    {

        StringBuilder builder = new StringBuilder(text.Length);

        for (int i = text.Length - 1; i >= 0; i--)

        {

            builder.Append(text[i]);

        }

​

        return builder.ToString();

    }

    public static string ReverseUsingCharacterBuffer(this string text)

        {

            char[] charArray = new char[text.Length];

            int inputStrLength = text.Length - 1;

            for (int idx = 0; idx <= inputStrLength; idx++) 

            {

                charArray[idx] = text[inputStrLength - idx];                

            }

            return new string(charArray);

        }

    public static string ReverseUsingStack(this string input)

        {

            Stack<char> resultStack = new Stack<char>();

            foreach (char c in input)

            {

                resultStack.Push(c);

            }

​

            StringBuilder sb = new StringBuilder();

            while (resultStack.Count > 0)

            {

                sb.Append(resultStack.Pop());

            }

            return sb.ToString();

        }

    public static string RecursiveReverse(this string str) {

                if (str.Length < 2) {return str;}

                return str[str.Length - 1] + RecursiveReverse(str.Substring(0, str.Length - 1));

        }

}