def largest_string(s: str) -> str:
    chars = list(s)
    length = len(chars)

    # instead of raising; this helper preserves that read behavior so the
    # lookahead checks below can't index past the end of the list.
    def char_at(idx: int) -> str:
        return chars[idx] if 0 <= idx < length else ""

    cur = ""
    i = length - 1
    while i >= 0:
        cur = char_at(i) + cur

        if len(cur) == 3:
            if cur == "abb":
                chars[i] = "b"
                chars[i + 1] = "a"
                chars[i + 2] = "a"
                if char_at(i + 4) == "b":
                    i += 4 + 1
                elif char_at(i + 3) == "b":
                    i += 3 + 1
                elif chars[i + 2] == "b":
                    i += 2 + 1
            if char_at(i + 1) == "b":
                i += 1 + 1
            else:
                i += 1
            cur = ""
        i -= 1

    return "".join(chars)