142: Letter case permutation

Given a string, return all possible strings that can be created by converting the letters to upper case and lower case

 

Example 1

Input:   "1a23"

Output:  ["1a23", "1A23"]

Example 2

Input:   "1a2A3"

Output:  ["1a2a3", "1a2A3", "1A2a3", "1A2A3"]