I need help coding a function that convert to and from the factorial number system in Java. These are the instructions:

The Factorial Number System, also called factoradic, is a mixed radix numeral system. The factorials determine the place value of a number.

In this system, the right most digit can be either 0 or 1, the second rightmost digit can be 0, 1 or 2, and so on. This means that an n digit factoradic number can have a maximum value of (n + 1)!.

For example, to convert the factoradic number 24201 to decimal you would do this:

2 * 5! = 240 4 * 4! = 96 2 * 3! = 12 0 * 2! = 0 1 * 1! = 1 240 + 96 + 12 + 0 + 1 = 349 

Hence the factoradic number 24201 is 349 base 10.

To convert a decimal number (with 349 as an example) into a factoradic number, you would do this:

Take the largest factorial less than the number. In this case it is 120, or 5!.

349 / 5! = 2 r 109 109 / 4! = 4 r 13 13 / 3! = 2 r 1 1 / 2! = 0 r 1 1 / 1! = 1 r 0 

Hence 349 base 10 is the factoradic number 24201.

Your challenge is to create the shortest program or function that converts an input number to the other base.

The input will be a string representation of a non-negative integer. A factoradic number will be preceded by a ! (eg. !24201), while a decimal number will not be preceded by anything. You may assume that the maximum input will be 10! - 1 - 3628799 in decimal and 987654321 in factoradic. This means that letters will not appear in a factoradic input/output.

The program doesn't need to prepend a ! to a factoradic output, and may output a string or an integer. The input may be in any reasonable format.

Test cases:

Input: 1234 Output: 141120 Input: 746 Output: 101010 Input: !54321 Output: 719 Input: !30311 Output: 381