Mathematical arrangement problem: "0, 1, 2, 3 constitute four non-repetitive numbers, how to arrange them?" Please make it clear, please.

First, choose thousands of digits, because it is the first digit, not 0, and you can only choose from 1, 2, 3. There are three possibilities.

If you choose one hundred digits, you can only choose from the remaining two digits of 1, 2 and 3,0. There are three possibilities.

If you choose ten digits, you can only choose from the remaining two digits 0, 1, 2, 3. There are two possibilities.

If you choose single digits, you can only choose from the remaining 1 digits of 0, 1, 2 and 3, and there are 1 possibilities.

therefore

* * * There are 3*3*2* 1= 18 non-repeated four digits.

They are:

1023, 1032, 1203, 1230, 1302, 1320,

20 13,203 1,2 103,2 130,230 1,23 10,

30 12,302 1,3 102,3 120,320 1,32 10