11. As shown in the figure on the right, there is a circle filled with the number 1. Please fill in the blank circles with one of the numbers $2,3,4,5,6$, ensuring no repetition, and the difference between numbers in adjacent circles is at least 2. How many different ways are there to fill the circles?
【Key Points】Counting, Enumeration
【Difficulty】Fufu
【Answer】 3
【Analysis】The difference between the numbers in adjacent circles is at least 2, so 2 can only be filled in $d$ and $e$.
(1) Fill 2 in $d$, 2's surroundings cannot have 3. So 3 can only be filled in $a$. 3's surroundings cannot have 4, so 4 can only be filled in $c$ and $e$. 5 and 6 cannot be adjacent, so 5 is filled in $b$. 6 and 4 can be exchanged in $c$ and $e$, at this point there are 2 ways to fill; (see middle figure)
(2) Fill 2 in $e$, 3 can be filled in $a$ or $b$.
If 3 is filled in $a$, 4, 5, 6 must have two adjacent, there is no way to fill that meets the condition;
If 3 is filled in $b$, 4 can only be filled in $c$, 5 can only be filled in $a$, 6 is filled in $d$.
1 way to fill; (see right figure)
Therefore, there are $2+1=3$ ways to fill in total.