In the first step, there was not interview, only a coding exercise containing 3 LeetCode -like questions. For one of them I had to think of the "fastest" solution, where for the rest I needed only to get correct algorithms.

Input: String in the format of XX, where X could be either X or a digit (guaranteed). Output: Fill the X's so the string will contain the "latest" date on the year in the format of DD, or null if the date is invalid. For example:

1. The input is "24", the XX will be 12 because the latest date in the year that could fit here is December 24th.
2. The output for "X1:11" the date would be "21:11" because November doesn't have only 30 days and not 31 so "31:11" is invalid
3. The output for "4X" would be null because there is no month with more than 31 days in it.

Note: Remember that each month has a different amount of days