You are given an integer `num`. You will apply the following steps exactly two times:

- Pick a digit `x (0 <= x <= 9)`.
- Pick another digit `y (0 <= y <= 9)`. The digit `y` can be equal to `x`.
- Replace all the occurrences of `x` in the decimal representation of `num` by `y`.
- The new integer cannot have any leading zeros, also the new integer cannot be 0.

Let `a` and `b` be the results of applying the operations to `num` the first and second times, respectively.

Return the max difference between `a` and `b`.

Example 1:

    Input: num = 555
    Output: 888
    Explanation: The first time pick x = 5 and y = 9 and store the new integer in a.
    The second time pick x = 5 and y = 1 and store the new integer in b.
    We have now a = 999 and b = 111 and max difference = 888

Example 2:

    Input: num = 9
    Output: 8
    Explanation: The first time pick x = 9 and y = 9 and store the new integer in a.
    The second time pick x = 9 and y = 1 and store the new integer in b.
    We have now a = 9 and b = 1 and max difference = 8

Constraints:

- `1 <= num <= 10`8