If the number of password digits is uncertain, please add up each case.

If the number cannot be duplicated:

One-digit password: 9 kinds;

Two-digit password: 9*8 kinds;

Three-digit password: 9*8*7 kinds;

Four-digit password: 9*8*7*6 kinds;

And so on;

If you can't determine the number of digits, add up each situation.

This is the principle of mathematical permutation and combination for your reference.