Remove adjacent duplicate groups repeatedly

Given a string s, repeatedly delete any maximal contiguous group of identical characters whose length is at least 2. After each deletion, the remaining parts concatenate and the process continues until no such group exists. Return the final string. Example: s = "abbba" → delete "bbb" → "aa" → delete "aa" → "". Design an O(n) time algorithm with O(n) extra space (e.g., using a stack-like technique), explain correctness, and analyze time/space complexity. Follow-up: generalize to delete groups of length ≥ k for a given k ≥ 2 while maintaining near-linear performance.