Minimize String with Insert Operation

Given a string s and an operation operation which allows inserting a character (selected from a given character set) at any position to make the string lexicographically smallest, write a function to return the smallest lexicographical string after the operation. Input A string s. A set of allowed characters for insertion operation. Output A string, representing the lexicographically smallest string after applying the operation. Constraints 1 <= len(s) <= 1000 operation contains at least one character Example Input abc x Output axbc Example Input abc x