How to reverse Word in a string Using 1 recursive Function (c-sharp)

Question

I'm facing a problem where I have to reverse words in string by 1 recursive Method. I have tries and achieved my result by using Two Method, out them 1 is recursive and other is iterative. But I couldn't find exact solution (i.e. by 1 recursive Method) anywhere. If you have any then please refer. Otherwise its really challenging! I want to reverse "Hello My World" to "olleH yM dlroW"

Note: As its a kind of homework, So I can't use built-in Functions or try other ways. I have to do it as I've described in details.

        static string Rev(string str)
        {
        if (str[0] == str[str.Length - 1])
        {
            return "";
        }
        if (str[0] == ' ')
        {
            return temp;
        }
        return temp += Rev(str.Substring(1)) + str[0];
        }

This above Method only returns the string in reverse which is before space, then it terminates. I've also tried that Method which reverse the whole string like "Hello World" to "dlroW olleH"

Answer 1

I want to reverse "Hello World" to "olleH dlroW"

Split the string on a space, and reverse each word. Then put the string back together with a space between each:

var input = "Hello world";
var result = String.Join(" ", 
                  input.Split(' ')
                       .Select(word => new string(word.Reverse().ToArray())));

Live example: https://rextester.com/CJF79825

Note that there are better and/or more efficient ways to reverse a string

Answer 2

There are probably many ways to do this; here's one example:

using System;
using System.Linq;

namespace Demo
{
    class Program
    {
        public static void Main(string[] args)
        {
            string text = "Hello World";
            Console.WriteLine(Reverse(text));  // Prints "olleH dlroW"
        }

        public static string Reverse(string text)
        {
            var words = text.Split(' ');

            return words.Length == 1 
                ? new string(words[0].Reverse().ToArray()) // Not recursive; using Enumerable.Reverse()
                : string.Join(" ", words.Select(Reverse)); // <- Here's the recursive call.
        }
    }
}

I have to say, though: This isn't the best question to illustrate recursion. I'd have gone for something more obvious such as calculating a factorial, or the Fibonacci sequence...

Answer 3

Looks like the actual question is how to reverse words in a string using recursion. Phrasing the question correctly is often 50% of actually solving it. Googling for reverse words with recursion returns several answers that show this is not an easy thing to do.

It requires two steps - one going forward to find the word boundaries and then one that backtracks to reverse the already identified word. I assume the point is to avoid loops entirely, so trying to find spaces using IndexOf is out of scope.

Reversing is easy - in fact the original code can be simplified a lot:

string Rev(string input)
{
    if (input.Length==0)
    {
        return input;
    }

    return Rev(input.Substring(1)) + input[0];
}

The problem with this function is that strings are immutable and every string operation allocates a new string that needs to be garbage-collected. It would be better to use Array.Reverse on the character array for this.

The second step is to find the words and transfer them from one call to the next. This similar question uses a stack to transfer the words, while this question splits a string into a head/tail when a space is discovered. The techniques can be combined, but instead of a stack we can use a string. Again, this wastes memory because it creates temporary strings.

string Rev(string input)
{
    if (input.Length==0)
    {
        return input;
    }

    return Rev(input.Substring(1)) + input[0];
}



string Word(string words,string head,string tail)
{   
    //Finished, return the final string.
    if(tail.Length==0)
    {
        return (words.Length==0)? Rev(head): words + " " + Rev(head);                
    }
    //Word boundary found
    if (tail[0]==' ')
    {
        //Add the revesed word to the words
        words=(words.Length==0)? Rev(head): words + " " + Rev(head);                
        head="";
        tail=tail.Substring(1);        
    }
    //No boundary, just copy one character from the tail to the head
    else
    {
        head=head + tail[0];
        tail=tail.Substring(1);       
    }

    return Word(words,head,tail);
}

Calling Word("","","Hello World Boo Soo") will return olleH dlroW ooB ooS.

Again, this is very wasteful.

Answer 4

You need to split the string into individual words, before you can reverse them, rather than treating it as a whole string.

The below example should achieve what you are looking for

class Program
{
    static string temp = string.Empty;

    static void Main(string[] args)
    {
        string word = "Hello World";
        var words = word.Split(' ');
        var result = ReverseWords(words);
        Console.Write(result);
        Console.ReadKey();
    }

    static string ReverseWords(string[] words)
    {
        var reversedList = new List<string>();
        foreach (var word in words)
        {
            temp = string.Empty;
            reversedList.Add(Rev(word, 1));
            reversedList.Add(" ");
        }
        return string.Join("", reversedList);
    }

    static string Rev(string str, int count)
    {
        if (str[0] == str[str.Length - count])
        {
            return temp += str[0];
        }
        return temp += Rev(str.Substring(count), count++) + str[0];
    }
}