To solve the problem, we construct the lexicographically largest string by selecting characters in descending order and limiting their consecutive repetitions. The challenge involves greedy choices and ensuring no letter appears more than repeatLimit times in a row. The solution can be efficiently achieved with a heap, hash table, and greedy strategies.
Problem Statement
You are given a string s and an integer repeatLimit. Your task is to construct a new string repeatLimitedString using the characters from s such that no letter appears more than repeatLimit times consecutively. You don't need to use all characters from s.
Return the lexicographically largest valid repeatLimitedString possible, ensuring the repeatLimit constraint is satisfied. A string a is lexicographically larger than string b if at the first differing position, a has a letter that comes later in the alphabet than b.
Examples
Example 1
Input: s = "cczazcc", repeatLimit = 3
Output: "zzcccac"
We use all of the characters from s to construct the repeatLimitedString "zzcccac". The letter 'a' appears at most 1 time in a row. The letter 'c' appears at most 3 times in a row. The letter 'z' appears at most 2 times in a row. Hence, no letter appears more than repeatLimit times in a row and the string is a valid repeatLimitedString. The string is the lexicographically largest repeatLimitedString possible so we return "zzcccac". Note that the string "zzcccca" is lexicographically larger but the letter 'c' appears more than 3 times in a row, so it is not a valid repeatLimitedString.
Example 2
Input: s = "aababab", repeatLimit = 2
Output: "bbabaa"
We use only some of the characters from s to construct the repeatLimitedString "bbabaa". The letter 'a' appears at most 2 times in a row. The letter 'b' appears at most 2 times in a row. Hence, no letter appears more than repeatLimit times in a row and the string is a valid repeatLimitedString. The string is the lexicographically largest repeatLimitedString possible so we return "bbabaa". Note that the string "bbabaaa" is lexicographically larger but the letter 'a' appears more than 2 times in a row, so it is not a valid repeatLimitedString.
Constraints
- 1 <= repeatLimit <= s.length <= 105
- s consists of lowercase English letters.
Solution Approach
Greedy Approach
Start by sorting the characters of the string in descending order. Then, use a greedy approach to construct the result string, ensuring that no character appears more than repeatLimit times in a row.
Heap for Efficient Character Selection
To efficiently manage the selection of characters, use a max heap (priority queue). This allows for quick access to the lexicographically largest character available that hasn't been used more than repeatLimit times consecutively.
Validation of Constraints
As you construct the string, continuously check that the number of consecutive repetitions of any character does not exceed repeatLimit. If it does, adjust the selection accordingly.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | O(N \cdot \log K) |
| Space | O(K) |
The time complexity of the solution is O(N * log K), where N is the length of the string and K is the number of unique characters. The space complexity is O(K), as we use a hash table or heap to store the character frequencies and manage the greedy selection process.
What Interviewers Usually Probe
- Can the candidate explain how a greedy approach ensures the largest lexicographical string while respecting the repeatLimit?
- Does the candidate recognize the importance of using a max heap for efficient character selection?
- Is the candidate able to demonstrate an understanding of the trade-offs between greedy strategies and maintaining constraints?
Common Pitfalls or Variants
Common pitfalls
- Forgetting to handle the repeatLimit constraint when appending characters, leading to invalid strings.
- Overcomplicating the problem by not using the heap or hash table efficiently for character management.
- Misunderstanding the lexicographical order, leading to suboptimal solutions where smaller characters are selected first.
Follow-up variants
- What if the repeatLimit was higher than the length of the string? How would the solution change?
- How does this solution handle edge cases like an empty string or a string where all characters are the same?
- What if the characters in the string are already sorted? How would that impact the approach?
How GhostInterview Helps
- GhostInterview provides an in-depth breakdown of greedy algorithms and explains how to handle constraints like repeat limits.
- It assists by guiding you through the use of data structures like heaps and hash tables for efficient problem-solving.
- GhostInterview helps identify common mistakes and traps, ensuring you stay focused on both efficiency and correctness during the interview.
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 best way to approach the "Construct String With Repeat Limit" problem?
The best approach is to use a greedy strategy, starting with the lexicographically largest characters and ensuring the repeatLimit constraint is respected through a heap or hash table.
How do I handle characters that appear more than repeatLimit times?
When a character appears more than the allowed repeatLimit, adjust the number of repetitions or skip to the next valid character, ensuring the result remains valid.
What is the time complexity of this problem?
The time complexity is O(N * log K), where N is the length of the string and K is the number of unique characters in the string.
What makes the solution greedy?
The solution is greedy because it always picks the lexicographically largest available character while ensuring the repeatLimit constraint is met.
Can you explain the use of the heap in this problem?
The heap is used to efficiently select the largest character available for string construction. It ensures that at each step, we can quickly access the next valid character to append.
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