## What is it? Find the longest substring where every letter appears in both uppercase and lowercase. A "nice" substring is one that is case-complete for all letters it contains. ## How it works **Divide and Conquer:** - Find any character in the string that appears in only one case (no counterpart) - That character cannot appear in any valid nice substring — split the string at that character - Recursively find the longest nice substring in each part - Return the longest result across all parts **Brute Force (O(n²)):** - Check every substring; for each, verify if it is "nice" using a set ## When to use - Problems involving character case constraints in substrings - Divide-and-conquer pattern practice ## Key Points - Divide-and-conquer runs in O(n²) worst case but is elegant and easy to implement - A nice substring must contain only characters that have both cases present - Handles empty strings and single-character strings trivially (return "")