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```2/22/17
BASIC LINKED DATA
STRUCTURES
Dr. John Georgas, Northern Arizona University
Copyright © John Georgas All Rights Reserved
Outline
2
¨
Current
¤ Data
structures, in general
¤ Arrays
¤ Linked
list
¤ Stack
¤ Queue
Data structures
3
¨
¨
Data structures define a way of storing and
organizing data elements
Data structures set rules for:
¤ What
kind of data is stored?
is data stored?
¤ How can data be accessed?
¤ How can data be inserted?
¤ How can data be removed?
¤ How
¨
Each data structure captures a set of design
decisions about these (and other) questions
1
2/22/17
Python lists and arrays
4
¨
¨
¨
Fundamental structure: Array
¤ Stores data that is the same size
¤ Organized contiguously
¤ Accessible in random order
¤ Fixed sized structure, unless resized
Python lists are built on top of dynamically sized arrays, as an array of
references
¤ References to objects are stored in the array
¤ Objects are stored externally on the heap
Complexity implications
¤ Reference of an index location?
¤ Value change?
¤ Addition of a list element?
¤ Removal of a list element?
Linked List
5
¨
Linked list consists of:
¤ Nodes:
each containing data and a reference to
another node
¤ Structure around nodes: allowing access into sequence
of nodes
¨
It’s all about trade-offs:
¤ Insertion
and removal is cheap
not support random access well (comparatively
expensive)
¤ Does
Variations, bells and whistles
6
¨
¨
¨
¨
Singly-linked list
¤ Structure: Nodes have one reference to the next node
¤ Function: Can only traverse one way
Doubly-linked list
¤ Structure: Nodes have two references, one to the next node and one to the
previous node
¤ Function: Can traverse either way
First and last node references
Counter
¤ A counter of nodes
n
¨
Amortization: Costs of algorithms is distributed among many operations
Circular-linked list
¤ Structure: Last node points back to the first
¤ Function: Complete iteration from any start position
2
2/22/17
Operations of interest, in Python
7
¨
¨
Linked lists, the OO way
¤
LinkedList
n __init_
n add_first, remove_first
n __len__
n __str__
¤
Node
n __init__
n __str__
¤
Iterators
n __init_
n next and StopIteration
n __iter__ from LinkedList
Using a linked list
¤
Creating a new linked list
¤
Adding some elements
¤
Remove some elements
¤
Traverse the linked list with an iterator
Stacks
8
¨
Elements of a stack can only be added to and
removed from the top
¤ Bottom
¨
can not be directly modified
Operations:
¤ pop
– remove element from the top of the stack
¤ push
– add element to the top of the stack
¨
Imposes a last-in, first-out ordering (LIFO)
¨
Built on top of a list, with a limited set of operations
¤ Or,
first-in, last-out
Queues
9
¨
Elements of a queue:
¤
¤
¨
Can only be added to the tail
Can only be removed from the head
Operations:
¤
¤
enqueue – add an element to the tail of the queue
dequeue – remove an element from the head of the queue
¨
Imposes a last-in, last-out ordering (LILO)
¨
Built on top of a list, with a limited set of operations
Variations:
¤
¨
¤
Or, first-in, first-out
Priority queue allows nodes to be marked with a priority
indicator, that overrides traditional ordering
3
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