Given a string `s`, you may transform each **letter** to lowercase or uppercase (digits stay unchanged). Return a list of all strings that can be created this way.
Results follow the order in which choices are explored (lowercase before uppercase at each letter).
Example 1
Input:s = "a1b2"
Output:["a1b2","a1B2","A1b2","A1B2"]
Explanation:Each letter toggles case independently.
Constraints
- 1 <= s.length <= 12
- s consists of letters and digits