Reduce the string by removing K consecutive identical characters

Maintain a stack of (character, count) runs while scanning the string. If the next character matches the top run, increment its count; otherwise push a new run. When a run reaches exactly k, remove it immediately. That removal is important because it can make equal runs from either side adjacent, and subsequent scanning naturally merges them. For example, with k=3, removing bbb in abbba leaves two a runs that should combine. If the variant removes groups of at least k rather than exactly k, clarify the rule: immediate deletion at k is equivalent for consecutive identical input under the usual cascading semantics, but wording matters. Avoid repeatedly rebuilding strings, which can be quadratic. Test k=1, an empty string, no removable run, runs longer than k, and multiple cascades. Each character enters and leaves the stack at most once, giving O(n) time and O(n) space in the worst case.