Formatted question description: https://leetcode.ca/all/1209.html

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

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.

Related Topics:
Stack

Similar Questions:

## Solution 1. Stack

// OJ: https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string-ii/
// 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 (int i = 0; i < st.size(); ++i) {
for (int j = 0; j < st[i].second; ++j) ans += st[i].first;
}
return ans;
}
};


Java

• class Solution {
public String removeDuplicates(String s, int k) {
StringBuffer sb = new StringBuffer();
List<Integer> duplicatesCounts = new ArrayList<Integer>();
int length = s.length();
for (int i = 0; i < length; i++) {
int curLength = sb.length();
char c = s.charAt(i);
int curCount = 1;
if (curLength > 0 && c == sb.charAt(curLength - 1))
curCount = duplicatesCounts.get(curLength - 1) + 1;
if (curCount == k) {
int startIndex = curLength - k + 1;
sb.delete(startIndex, curLength);
for (int j = curLength - 1; j >= startIndex; j--)
duplicatesCounts.remove(j);
} else {
sb.append(c);
}
}
return sb.toString();
}
}

• // OJ: https://leetcode.com/problems/remove-all-adjacent-duplicates-in-string-ii/
// 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;
}
};

• # 1209. Remove All Adjacent Duplicates in String II

class Solution:
def removeDuplicates(self, s: str, k: int) -> str:
stack = []

for c in s:
if stack and stack[-1] == c:
ch, cnt = stack.pop()
stack.append((ch, cnt + 1))
else:
stack.append((c, 1))

if stack and stack[-1] == k:
stack.pop()

return "".join(ch * cnt for ch, cnt in stack)