How to Compress a String using Recursion? (RLE algorithm)

Question

I'm having slight trouble trying to compress a string using recursion.

For example, consider the following string:

qwwwwwwwwweeeeerrtyyyyyqqqqwEErTTT

After applying the RLE algorithm, this string is converted into:

q9w5e2rt5y4qw2Er3T

In the compressed string, "9w" represents a sequence of 9 consecutive lowercase "w" characters. "5e" represents 5 consecutive lowercase "e" characters, etc.

I already have a code for compressing it without recursion:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Compress {

    public static String input(String source) {
        StringBuffer coding = new StringBuffer();
        for (int i = 0; i < source.length(); i++) {
            int runLength = 1;
            while (i+1 < source.length() && source.charAt(i) == source.charAt(i+1)) {

                runLength++;   

                i++;

           }
            if (runLength>1){
            coding.append(runLength);
            }
            coding.append(source.charAt(i));
        }
        return coding.toString();
    }

    public static void main(String[] args) {

        IO.outputStringAnswer("Enter a string");
        String str = IO.readString();
        String result = ""; 
        result=input(str); //function(variable)


        IO.outputStringAnswer(result);
    }
}

But I am unsure if this is able to be turned into a recursion version of this.

Answer 1

This is most likely what you are looking for:

public static String compress(String source) {
    if (source.length() <= 1) return source;

    int runLength = 1;
    while (runLength < source.length() && source.charAt(0) == source.charAt(runLength)) {
        runLength++;
    }

    String lengthString = runLength > 1 ? String.valueOf(runLength) : "";
    return lengthString + source.substring(0,1) + compress(source.substring(runLength));
}

I assume your source String does not contain any digits. As you can see the function calls itself recursively in the last line with the remainder of the source string.