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In a singly linked list, we can add to the head of the list by checking to see if it already has a head. We then either set the new node as the head (if there was no head) or update the head property, and link the past head to the new head.
Since a doubly linked list has an additional tail property and is built with nodes that each have two pointers, there are a few more steps:
- Start by checking to see if there is a current head to the list
- If there is (meaning the list is not empty), then we want to reset the pointers at the head of the list:
- Set the current head’s previous node to the new head
- Set the new head’s next node to the current head
- Update the head property to be the new head
- Finally, if there isn’t a current tail to the list (meaning the list was empty):
- Update the tail property to be the new head since that node will be both the head and tail of the list
Instructions
1.
Define an .add_to_head() method that takes self and new_value as parameters.
Inside, create:
- A
new_headNodethat takesnew_valueas a parameter - A
current_headNodethat’s set to the list’s head
2.
If there is a current head to the list:
- Set
current_head‘s previous node tonew_head - Set
new_head‘s next node tocurrent_head
Remember to use the Node class’s .set_prev_node() and .set_next_node() methods.
3.
Outside of the if statement, set the list’s head to new_head.
4.
Lastly, if the list doesn’t have a tail, set the list’s tail to the new head.
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