LeetCode 784 Letter Case Permutation - Problem Statement

Given a string, return all possible strings where each letter can be lowercase or uppercase.

With "a1b2":

• 'a' can be 'a' or 'A'
• 'b' can be 'b' or 'B'
• Digits stay the same

Result: ["a1b2", "a1B2", "A1b2", "A1B2"].

Non-letter characters remain unchanged.

Constraints: $1 \le$ length $\le 12$. String contains letters and digits only.