Determine if an input string matches a given regular expression that supports '.' and '*' characters. The '.' matches any single character, and '*' matches zero or more occurrences of the preceding element.

Given a string of digits, remove *k* digits to obtain the smallest possible number. You need to determine which digits to remove to minimize the resulting number.

You have K sorted lists of numbers. Find the smallest range [min, max] that includes at least one number from each of the K lists.