Analysis: one, four people and four cards number ABCD abcd. First, take A as the research object. A can only give A to B, C and D, and one of the three people has C(3, 1) (can't give it to himself). Suppose A gives A to C, then C can give C to one of A, B and D 1) Suppose C gives B two cards, B and D, A and D, then there is only one delivery method, that is, B gives D and D to A. Suppose the delivery method: A → C; c→B； b→D； D → A. It can be seen that according to the principle of step-by-step counting, there are 3*3=9 kinds of * *.

[Attached]:

1:

a→B b→A c→D d→D

Type 2:

a→B b→C c→D d→A

The third kind:

a→B b→D d→C c→A

The fourth kind:

a→C c→A b→D d→B

Type 5: a → c c → bb → DD → a.

Type 6:

a→C c→D d→B b→A

Type 7:

a→D d→A b→C c→B

Type 8:

a→D d→B b→C c→A

Type 9:

a→D d→C c→B b→A

Note: the assumptions in the analysis can be summarized and can be understood from the attached book!

I hope my answer will satisfy you!