Start by iterating through the string and using a stack to track characters. Whenever AB or CD appears on top of the stack, remove them immediately. This guarantees minimal string length efficiently without brute-force recursion and handles all combinations systematically.
Problem Statement
You are given a string s containing only uppercase English letters. You may perform operations where any occurrence of the substring AB or CD can be removed in a single step.
Return the minimum possible length of s after applying these operations any number of times. The order of removals may affect intermediate states but the final length should be minimized.
Examples
Example 1
Input: s = "ABFCACDB"
Output: 2
We can do the following operations:
- Remove the substring "ABFCACDB", so s = "FCACDB".
- Remove the substring "FCACDB", so s = "FCAB".
- Remove the substring "FCAB", so s = "FC". So the resulting length of the string is 2. It can be shown that it is the minimum length that we can obtain.
Example 2
Input: s = "ACBBD"
Output: 5
We cannot do any operations on the string so the length remains the same.
Constraints
- 1 <= s.length <= 100
- s consists only of uppercase English letters.
Solution Approach
Stack Simulation
Iterate through each character and push it onto a stack. If the top two elements form AB or CD, pop them immediately. Continue until the string is fully processed.
Avoid Brute Force
Directly checking all removal combinations is inefficient. The stack-based approach automatically handles overlapping patterns and ensures minimal string length without exponential checks.
Track State Carefully
Always check the top of the stack after each insertion to see if a removable substring has formed. Missing this leads to incorrect lengths for certain input sequences.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | O(n) |
| Space | O(n) |
Time complexity is O(n) since each character is pushed and popped at most once. Space complexity is O(n) for the stack storing intermediate characters.
What Interviewers Usually Probe
- Mentions repeated removal of substrings AB or CD.
- Hints at using a stack to maintain current string state.
- Asks about handling overlapping patterns efficiently.
Common Pitfalls or Variants
Common pitfalls
- Trying brute-force removal of all substring combinations instead of stack-based tracking.
- Failing to check the top of the stack after each character push.
- Assuming non-overlapping removals are sufficient, missing cases where removals create new AB or CD sequences.
Follow-up variants
- Remove different fixed-length substrings beyond AB or CD using the same stack method.
- Allow removal of substrings with variable lengths but fixed patterns.
- Compute the minimum string length if only one type of substring is removable.
How GhostInterview Helps
- Automatically suggests stack-based simulation for substring removal sequences.
- Highlights patterns where naive removal fails and guides efficient handling.
- Provides step-by-step reasoning to verify minimal resulting string length.
Topic Pages
Related GhostInterview Pages
- LeetCode Interview Copilot - Use GhostInterview as a live solver when you want direct help with LeetCode-style coding questions.
- Coding Interview Assistant - See how GhostInterview supports array, string, linked list, graph, and tree interview workflows.
- How GhostInterview Works - Review the screenshot, reasoning, and answer flow before using the solver in a live interview.
FAQ
What is the main strategy for Minimum String Length After Removing Substrings?
Use a stack to push characters and remove AB or CD whenever they appear on top, ensuring minimal string length efficiently.
Can we solve this problem with brute force?
Brute force is possible but inefficient; it cannot scale due to exponential removal combinations. Stack simulation is optimal.
Why do we check only the top of the stack for removals?
Because only the last inserted characters can form AB or CD with the current character, ensuring correct minimal length.
What is the time and space complexity?
Time complexity is O(n) and space complexity is O(n), where n is the length of the input string.
Does the order of removals affect the final string length?
Stack simulation ensures all necessary removals are applied in order, so the final minimal length is achieved regardless of intermediate choices.
Need direct help with Minimum String Length After Removing Substrings instead of spending more time grinding it?
Download GhostInterview when you want a LeetCode solver, not another long practice loop. Capture Minimum String Length After Removing Substrings from a screenshot, get the answer path and complexity, and use supported stealth workflows that stay outside captured layers.
Capture the prompt fast instead of rewriting the problem by hand.
Get the solution path, trade-offs, and complexity summary in one pass.
Stay outside captured layers on supported screen-share workflows.
Stay in the same pattern family
Remove all stars from a string by simulating the removal process using a stack to manage characters and stars efficiently.
Open problem page#2296 Design a Text EditorDesign a text editor that supports text manipulation and cursor navigation operations efficiently with linked-list-based data structures.
Open problem page#3174 Clear DigitsRemove all digits from a given string by applying a repeated operation to form the final string without digits.
Open problem page