java Run-length encoding

Question

I have no idea how to start my assignment.

We got to make a Run-length encoding program,

for example, the users enters this string:

aaaaPPPrrrrr

Answer 1

This can easily and simply be done using a StringBuilder and a few helper variables to keep track of how many of each letter you've seen. Then just build as you go.

For example:

static String encode(String s) {
    StringBuilder sb = new StringBuilder();
    char[] word = s.toCharArray();
    char current = word[0]; // We initialize to compare vs. first letter

           // our helper variables
    int index = 0; // tracks how far along we are
    int count = 0; // how many of the same letter we've seen

    for (char c : word) {
        if (c == current) {
            count++;
            index++;

            if (index == word.length)
                sb.append(current + Integer.toString(count));
        }

        else {
            sb.append(current + Integer.toString(count));
            count = 1;
            current = c;
            index++;
        }
    }
    return sb.toString();
}

Since this is clearly a homework assignment, I challenge you to learn the approach and not just simply use the answer as the solution to your homework. StringBuilders are very useful for building things as you go, thus keeping your runtime O(n) in many cases. Here using a couple of helper variables to track where we are in the iteration "index" and another to keep count of how many of a particular letter we've seen "count", we keep all necessary info for building our encoded string as we go.