1209. Remove All Adjacent Duplicates in String II

Given a string s, 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 <= 10^5
• 2 <= k <= 10^4
• s only contains lower case English letters.

Solutions (Rust)

1. Stack

impl Solution {
    pub fn remove_duplicates(s: String, k: i32) -> String {
        let mut stack: Vec<(u8, i32)> = vec![];

        for ch in s.bytes() {
            match stack.last_mut() {
                Some(last) if ch == last.0 => {
                    last.1 += 1;
                    if last.1 == k {
                        stack.pop();
                    }
                }
                _ => stack.push((ch, 1)),
            }
        }

        String::from_utf8(
            stack
                .into_iter()
                .map(|(ch, cnt)| vec![ch; cnt as usize])
                .flatten()
                .collect(),
        )
        .unwrap()
    }
}