OK, a sequel!

I like:

1. We have two views of the same array.
2. We can start at 1 in the for-loop
3. Opportunity for invariants to help our loop-based reasoning.
4. Things get much simpler with the helper function (i.e., I made a lot of bugs until I decided to use a helper function).

I don’t think I’m biased to say this one requires a bit of careful thought!

class Solution(object):
    def commitLetter(self, chars, c, i, count):
        assert(count > 0)
        chars[i] = c
        i += 1
        if count > 1:
            w = str(count)
            for t in w:
                chars[i] = t
                i += 1
        return i
            
    def compress(self, chars):
        if len(chars) == 0: return []
        
        count = 1
        j = 0
        lastChar = chars[0]
        
        for i in range(1, len(chars)):
            if chars[i] == lastChar:
                count += 1
            else:
                j = self.commitLetter(chars, lastChar, j, count)
                lastChar = chars[i]
                count = 1
        j = self.commitLetter(chars, lastChar, j, count)
        return j