Remove All adjacent Duplicates in String II

Jin Shang bio photo By Jin Shang

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"
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"


  • 1 <= s.length <= 10^5
  • 2 <= k <= 10^4
  • s only contains lower case English letters.


Iterate from left to right, maintain a cound of how many consecutive chars we have at position i and a result string. Whenever the count reaches k we remove k elements.

class Solution {
    string removeDuplicates(string s, int k) {
        string cur = "";        
        vector<int> dp(s.size()+1, 0);
        for(int i = 1; i<=s.size();i++){
                cur += s[i-1];
                dp[i] = 1;
                if(cur[cur.size()-1] == s[i-1]){
                    cur += s[i-1];
                    dp[cur.size()] = dp[cur.size()-1]+1;
                    if(dp[cur.size()] == k){
                        cur = cur.substr(0,cur.size()-k);
                    cur += s[i-1];
                    dp[cur.size()] = 1;
            // cout<<cur<<" "<<dp[cur.size()]<<"\n";
        return cur;