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

Input: S = "3z4" Output: ["3z4", "3Z4"]

Input: S = "12345" Output: ["12345"] Note:

S will be a string with length at most 12. S will consist only of letters or digits.

Solution: O(N x 2 ^ N) Backtrack and generate all possible combinations.