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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:

• Remove All Adjacent Duplicates In String (Easy)

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
• C++
• Python
• Go

• 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);
                  duplicatesCounts.add(curCount);
              }
          }
          return sb.toString();
      }
  }
  
  ############
  
  class Solution {
      public String removeDuplicates(String s, int k) {
          Deque<int[]> stk = new ArrayDeque<>();
          for (int i = 0; i < s.length(); ++i) {
              int j = s.charAt(i) - 'a';
              if (!stk.isEmpty() && stk.peek()[0] == j) {
                  stk.peek()[1] = (stk.peek()[1] + 1) % k;
                  if (stk.peek()[1] == 0) {
                      stk.pop();
                  }
              } else {
                  stk.push(new int[] {j, 1});
              }
          }
          StringBuilder ans = new StringBuilder();
          for (var e : stk) {
              char c = (char) (e[0] + 'a');
              for (int i = 0; i < e[1]; ++i) {
                  ans.append(c);
              }
          }
          ans.reverse();
          return ans.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;
      }
  };
  

• class Solution:
      def removeDuplicates(self, s: str, k: int) -> str:
          stk = []
          for c in s:
              if stk and stk[-1][0] == c:
                  stk[-1][1] = (stk[-1][1] + 1) % k
                  if stk[-1][1] == 0:
                      stk.pop()
              else:
                  stk.append([c, 1])
          ans = [c * v for c, v in stk]
          return "".join(ans)
  
  ############
  
  # 1209. Remove All Adjacent Duplicates in String II
  # https://leetcode.com/problems/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][0] == c:
                  ch, cnt = stack.pop()
                  stack.append((ch, cnt + 1))
              else:
                  stack.append((c, 1))
              
              if stack and stack[-1][1] == k:
                  stack.pop()
          
          return "".join(ch * cnt for ch, cnt in stack)
  
  

• func removeDuplicates(s string, k int) string {
      stk := []pair{}
      for _, c := range s {
          if len(stk) > 0 && stk[len(stk)-1].c == c {
              stk[len(stk)-1].v = (stk[len(stk)-1].v + 1) % k
              if stk[len(stk)-1].v == 0 {
                  stk = stk[:len(stk)-1]
              }
          } else {
              stk = append(stk, pair{c, 1})
          }
      }
      ans := []rune{}
      for _, e := range stk {
          for i := 0; i < e.v; i++ {
              ans = append(ans, e.c)
          }
      }
      return string(ans)
  }
  
  type pair struct {
      c rune
      v int
  }
  

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