Letter Case Permutation

Given a string containing letters and digits, return all possible strings formed by independently toggling the case of each letter. Digits remain unchanged. Return the results in any order.

Examples
Input: "a1b2"
Output: ["a1b2","a1B2","A1b2","A1B2"]
Hints

Letter Case Permutation

Given a string containing letters and digits, return all possible strings formed by independently toggling the case of each letter. Digits remain unchanged. Return the results in any order.