1209. remove all adjacent duplicates in string ii

You are given a string s and an integer k, a k duplicate removal consists of choosing k adjacent and equal letters from s and removing them, causing the left and the right side of the deleted substring to concatenate together.

We repeatedly make k duplicate removals on s until we no longer can.

Return the final string after all such duplicate removals have been made. It is guaranteed that the answer is unique.

solution

• we keep pushing each new character into a stack, and delete all blocks of k characters as they come up.
• we store tuples of [char, count] instead of just characters in order to improve the speed of checking for blocks from $O(n)$ to $O(1)$.

def removeDuplicates(self, s: str, k: int) -> str:
	stack = []
	for char in s:
		if not stack or stack[-1][0] != char:
			stack.append([char, 1])
		else:
			stack[-1][1] += 1
			if stack[-1][1] == k:
				stack.pop()
	return "".join(char*freq for char, freq in stack)