Stacks and Queues
Stacks and Queues
• Not really “data structures”
– More of an enforcement of policy
– Can be implemented using an array or linked list
– Can store just about any kind of data
• Queues
– First In, First Out (FIFO)
– Like waiting in line
• Stacks
– First In, Last Out (FILO)
– Like a stack of trays
Stacks
• Three primary operations
– Push() – put new data on the top of the stack
– Pop() – remove data from the top of the stack
– Peek() – get a copy of the data on the top of the
stack
• Useful
– our function stack!
– stack of cards, tiles, loot, etc…
Example
(pushing)
5
Note: could use a linked list also
Example
(pushing)
11
5
Example
(pushing)
-6
11
5
Example
(pushing)
45
-6
11
5
Example
(current stack)
45
-6
11
5
Example
(peeking)
45
45
-6
11
5
Example
(popping)
45
-6
11
5
Example
(pushing)
31
-6
11
5
Example
(popping)
31
-6
11
5
Example
(popping)
-6
11
5
Example
(popping)
11
5
Example
(popping)
FILO
5
Queues
• Two primary operations
– Enqueue() – put new data at the end of the queue
– Dequeue() – remove data from the beginning of
the queue
– Peek() – yes, it’s still there…
• Useful
– Enforcing fairness (waitlist at SPSU)
– Player turns during a round
Example
Example
(enqueue a 5)
5
Example
5
Example
(enqueue an 11)
5
11
Example
5 11
Example
(enqueue a -6)
5 11
-6
Example
5 11 -6
Example
(dequeue)
5 11 -6
Example
(dequeue)
5 11 -6
Example
(dequeue)
5
11 -6
FIFO
Example
(dequeue)
11 -6
Summary
• Not really “data structures”
– More of an enforcement of policy
– Can be implemented using an array or linked list
•
•
•
•
Queues are FIFO
Stacks are FILO
Which data structure is LILO?
Which one is LIFO?