Remove Repeated Character Groups

You are given a string `s` and an integer `k`. Repeatedly remove any contiguous group of `k` identical characters. After a removal, the remaining parts of the string are concatenated, which may create new removable groups. Continue until no more removals are possible, and return the final string. As a warm-up, explain how you would solve the special case `k = 3`, then generalize the approach to any `k`. Constraints: - `1 <= len(s) <= 100000` - `2 <= k <= 100000` - `s` contains lowercase English letters Example: - Input: `s = "abbbaaac"`, `k = 3` - Output: `"c"`