Doubly linked lists Computer science principles

1. Doubly linked lists
Eleonora Ciceri, Politecnico di Milano
Email: eleonora.ciceri@polimi.it

2. Doubly linked lists
 A doubly linked list is a list that can be traversed both from
left to right and from right to left
 It requires to store two head pointers:
 The first one points to the first element in the list
 The second one points to the last element in the list
first
last

3. Linked lists vs. Doubly linked lists
 Linked lists
 Simple implementation
 Require less memory
 Doubly linked lists
 More complex implementation
 Require more memory
 Accessing elements is easier, since accessing from the “head” (i.e.,
leftmost element) costs as much as accessing from the “tail” (i.e.,
rightmost element)
 Worst case: accessing the middle element (it requires to go
through half list, both from left and from right)

4. Creating and printing doubly linked
lists

5. Node definition
 Each node contains:
 The data
 A pointer to the previous element (left-side)
 A pointer to the next element (right-side)
previousPtr data nextPtr

6. Creating a doubly linked list
 We are going to create a doubly linked list by hand:
first second last

7. second last
Creating a doubly linked list
 We are going to create a doubly linked list by hand:
first

8. second last
Creating a doubly linked list
 We are going to create a doubly linked list by hand:
first

9. Print the list starting from the head
 Strategy
 Start from the head (i.e., the leftmost element)
 Use the nextPtr pointers to go through the list, from left to right

10. Print the list starting from the tail
 Strategy
 Start from the tail (i.e., the rightmost element)
 Use the previousPtr pointers to go through the list, from right to
left

11. References

12. References
 Doubly linked list:
http://www.youtube.com/watch?v=k0pjD12bzP0