A string is called an increasing list if it is a comma-separated list of increasing positive integers with no leading zeroes. For example, the strings "2,3,9", "30", and "1,100,1000000000000000000000" are increasing lists, while "5,6,6", "1,2,3,", "0" and "1,02" are not.
You're given a String mask consisting of digits, commas and question marks.
Replace each question mark with a digit or a comma, so that the resulting string is an increasing list, and return the resulting string.
If there are multiple possible resulting strings, return the lexicographically smallest one.
If it is impossible to produce an increasing list, return the string "impossible" (quotes for clarity only).