Problem Statement

Given a string str, return the longest nice substring of a given string.

A substring is considered nice if for every lowercase letter in the substring, its uppercase counterpart is also present, and vice versa.

If no such string exists, return an empty string.

Examples

1. Example 1:

   • Input: "BbCcXxY"
   • Expected Output: "BbCcXx"
   • Justification: Here, "BbCcXx" is the longest substring where each letter's uppercase and lowercase forms are present.

2. Example 2:

   • Input: "aZAbcD"
   • Expected Output: ""
   • Justification: There is no contiguous substring where each character exists in both its uppercase and lowercase forms.

3. Example 3:

   • Input: "qQwWeErR"
   • Expected Output: "qQwWeErR"
   • Justification: The entire string is the longest nice substring since every letter exists in both uppercase and lowercase forms.

Constraints:

• 1 <= s.length <= 100
• s consists of uppercase and lowercase English letters.

Try it yourself

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