Remove All Adjacent Duplicates in String II

Problem Description

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.

Example 1:

Input: s = "abcd", k = 2
Output: "abcd"
Explanation: There's nothing to delete.

Example 2:

Input: s = "deeedbbcccbdaa", k = 3
Output: "aa"
Explanation: 
First delete "eee" and "ccc", get "ddbbbdaa"
Then delete "bbb", get "dddaa"
Finally delete "ddd", get "aa"

Example 3:

Input: s = "pbbcggttciiippooaais", k = 2
Output: "ps"

Constraints:

• 1 <= s.length <= 105
• 2 <= k <= 104
• s only contains lowercase English letters.

Solution (JavaScript)

/**
 * @param {string} s
 * @param {number} k
 * @return {string}
 */
var removeDuplicates = function (s, k) {
    const stack = [s[0]];
    const count = [1];
    for (let i = 1; i < s.length; i++) {
        if (s[i] === stack[stack.length - 1]) {
            if (count[count.length - 1] + 1 === k) {
                stack.pop();
                count.pop();
            } else {
                count[count.length - 1] += 1;
            }
        } else {
            stack.push(s[i]);
            count.push(1);
        }
    }
    let r = "";
    for (let i = 0; i < stack.length; i++) {
        r += stack[i].repeat(count[i]);
    }
    return r;
};