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"

Remove Repeated Character Groups

You are given a lowercase string s and an integer k.

Repeatedly remove any contiguous group of at least 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.

Warm-up: first think about the special case k = 3. If a middle group disappears, two groups on its left and right may become adjacent and merge into a new removable group. Then generalize the same idea to any k.

Example: s = "abbbaaac", k = 3

• Remove bbb -> aaaa c
• Now aaaa is a group of length 4, which is at least 3, so remove it
• Final result: "c"

Hints

1. For the warm-up k = 3, think in terms of runs of equal characters rather than individual characters. Removing one run can cause neighboring runs to merge.
2. A stack of [character, count] pairs lets you track the compressed string built so far. When a merged run reaches length at least k, delete that whole run.