784. Letter Case Permutation 
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Problem
Given a string s, you can transform every letter individually to be lowercase or uppercase to create another string.
Return a list of all possible strings we could create. Return the output in any order.
Example 1:
Input: s = "a1b2" Output: ["a1b2","a1B2","A1b2","A1B2"]
Example 2:
Input: s = "3z4" Output: ["3z4","3Z4"]
Constraints:
1 <= s.length <= 12sconsists of lowercase English letters, uppercase English letters, and digits.