Write a method that returns a string representing an integer 'num' with the string 'str' inserted BETWEEN each digit of the number. Preconditions: 'num' will contain a positive integer and 'str' will contain a string (possibly an empty string). Return a string with the digits of 'num' with 'str' between each digit. If 'num' is a single-digit number, just return a string that represents that number since there are no spaces between digits in a single-digit number.