If we input a number formed by 4 digits and these digits are not all of one same value, then it obeys the following law. Let us operate the number in the following way:
(1) Arrange the digits in the way from bigger to smaller, such that it forms the biggest number that could be made from these 4 digits;
(2) Arrange the digits in the way from smaller to bigger, such that it forms the smallest number that could be made from these 4 digits (If there is 0 among these 4 digits, the number obtained may be less than four digits);
(3) Find the difference of these two numbers that is a new four digital number.
Repeat the above process, we can finally always get the result 6174 or 0.
Please write the program to realize the above algorithm.