stumped on a Java interview, need some hints

Question

This is an interview problem that I am stuck on:

Given a string consisting of a, b and c\'s, we can perform the following operation: Take any two adjacent

Answer 1

Edit For fun I did my own version that operates on a char[] in-place:

public class Main {

    public static void main(String[] args) {
        System.out.println(solve("abbccaacba"));
    }

    private static int solve(String testCase) {
        if (!testCase.matches("^[abc]*$"))
            throw new IllegalArgumentException("invalid input");

        char[] chars = new char[testCase.length()];
        testCase.getChars(0, testCase.length(), chars, 0);

        int remaining = chars.length;

        for (int i=0; (i1);)
        {
            int next = i+1;
            while (next < chars.length && (' '==chars[next]))
                ++next;

            if (next >= chars.length)
                break;

            if (chars[i]!=chars[next])
            {
                switch (chars[i])
                {
                    case 'a': chars[next] = ('b'==chars[next])? 'c':'b'; break;
                    case 'b': chars[next] = ('a'==chars[next])? 'c':'a'; break;
                    case 'c': chars[next] = ('b'==chars[next])? 'a':'b'; break;
                }
                chars[i] = ' '; // mark as removed
                remaining--;

                while (i>0 && (' '!=chars[i-1]))
                    --i;
                if (' '==chars[i])
                    i = next;
            }
            else
                ++i;
        }

        return remaining;
    }
}

See it live on http://ideone.com/yhK9t, with debug output:

a

Still do note caveats I mentioned in my comments: EDIT Huh, somehow I borked a comment saying that the answers would vary depending on the order of substitutions

• left to right or right-to-left (my version uses left-to-right)
• depth-first (or breadth-first) (my version uses depth-first)