1. Queue and
stacks
• What they are
• How they work
• How to make them
• Typical problems

2. Sample exam question
• What is a queue?
• What is a stack?
• How can one reverse the other?

3. Queue
• First in
• First Out
FIFO

4. Stack
• Last in
• First out
LIFO

5. The problem with
queues
• Queues work as you would expect
– First in, First out - FIFO
– Later items take their turn behind earlier
ones.
• Because of the ‘no shuffling’ rule,
these turn out to be tricky to
implement

6. Classic Array-Style Queue
• Items are read off the front of the
queue
W H E R E sp T H E sp
Front of Queue Rear of Queue Limit

7. Classic Array-Style Queue
• And added to the rear
W H E R E sp T H E sp S
Front of Queue Rear of Queue Limit
Item = W

8. Classic Array-Style Queue
• Until we run out of space
• Then we’re stuffed.
W H E R E sp T H E sp S T R E
Front of Queue
Rear of Queue
Limit
Item = WHER Warning: Queue Full!

10. Brilliant Idea!
• If you get to the end ..
• Wrap round to the front
• Until you hit the Rear pointer
E E T R E sp T H E sp S T R E
Front of QueueRear of Queue Limit
Item = WHERE

11. Circular queue
• It’s not really a circle!
E
E
T R E sp T H E sp S T
R
E
Front of Queue
Rear of Queue
EndStart