As part of a larger scale project, you need to write a component which generates consecutive positive integers. Only certain digits may appear in the input and in the integers generated, and leading zeros aren't allowed.
You are given a int allowed containing the list of allowed digits, and a String current representing the current integer. Return a String representing the first integer larger than current composed only of digits in allowed.
If current represents an invalid integer according to the first paragraph, return "INVALID INPUT" (quotes for clarity).