$2)$ If instead L is a doubly linked list, refer to the example code provided above, fill in the code blocks to

fulfill the wanted purposes. a$.$ Insert a node s after the $2$nd node $(5\%)$

b$.$ Insert a node s before the last node $(5\%)$

c$.$ Swap the positions of the first and the last node. Do not modify the values $($data$)$ of the nodes $(10\%)$