1209

1209. Remove All Adjacent Duplicates in String II (Medium)

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 lower case English letters.

Related Topics:
String, Stack

Similar Questions:

• Remove All Adjacent Duplicates In String (Easy)
• Replace Non-Coprime Numbers in Array (Hard)

Solution 1. Stack

// OJ: https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string-ii/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    string removeDuplicates(string s, int k) {
        vector<pair<char, int>> st;
        for (char c : s) {
            if (st.empty() || st.back().first != c) st.emplace_back(c, 1);
            else if (++st.back().second == k) st.pop_back();
        }
        string ans;
        for (auto &[c, cnt] : st) {
            while (cnt--) ans += c;
        }
        return ans;
    }
};