Write 9 lines ... I in letters A and B, representing 9 sets;

When you input it into the computer, you will find that for any two elements α and β, there must be m, n,

Make α∈M, β∈N and M∩N≠ empty sets,

Let γ∈M∩N, then the number of times you want to reverse from A to B is 3.

Because the line where A is located is the line represented by set A and the line where B is located is the line represented by set B, there must be transfer stations on A and B, so any two elements on A and B are α and β.

Then Line A (A) → Line M → Line N → Line B (B), * * * needs to transfer three times, and the transfer stations are α, γ and β respectively.

-Hangel