2182. Construct String With Repeat Limit - Detailed Explanation

Approach 2: Greedy with Max-Heap (Priority Queue) in Python

Idea: This approach is similar to Approach 1 but uses a max-heap to manage characters and their frequencies dynamically. We always pick the largest available character that does not violate the repeat limit.

Python Code:

Step-by-Step Walkthrough

1. Sorting Approach: Count the frequency of each character and sort characters in descending lexicographical order. Build the result string by repeatedly taking the highest-frequency available character. If a character is about to exceed the repeatLimit in consecutive occurrences, switch to the next largest character for one position, then continue with the first character again.

2. Priority Queue Approach: Use a max-heap to always fetch the currently largest character. Append it up to repeatLimit times. If it still has remaining occurrences, temporarily use one occurrence of the next largest character from the heap to break the sequence, then push the original character back into the heap to continue later.

Common Mistakes and Edge Cases

• Single-character string: If s consists of only one unique character, the output will just be that character repeated up to repeatLimit (or the length of s, whichever is smaller). There is no need to insert other characters.

• repeatLimit larger than any frequency: If repeatLimit is greater than the frequency of every character in s, then no character will ever repeat enough to violate the limit. In this case, a simple sorted (descending) arrangement of the string will already be valid.

• All characters are the same: When s has one character repeated (e.g., "aaaaa" with repeatLimit = 2), the result will include as many of that character as allowed in a row (two in this case), but then you cannot add another of the same character without breaking the rule. If no other character is available to insert in between, you simply stop (using as many characters as possible up to the limit each time).

Alternative Variations

1. Strings with Additional Constraints: You might encounter variations where you need to rearrange a string under different constraints, such as ensuring more complex patterns (for example, no two adjacent vowels, etc.). The greedy approach with sorting or heaps can often be adapted to these scenarios.

2. Maximizing Lexicographical Order: Sometimes the goal is to form the lexicographically largest (or smallest) string under certain constraints. In such cases, a similar greedy strategy is used, but careful consideration is needed to always choose the optimal next character while respecting the rules.

Related Problems for Further Practice

1. Rearrange String k-Distance Apart: Arrange characters so that identical characters are at least k distance from each other.

2. Rearrange Characters to Make String k-Distance Apart: A variation of the above problem, ensuring separation of identical characters.

3. Longest Happy String: Construct the longest possible string by repeatedly adding characters under the condition that no letter appears three times in a row. (This is a similar greedy problem on LeetCode where you must avoid long repeats.)