2. Recall: stacks and queues
stack: retrieves elements in reverse order as added
queue: retrieves elements in same order as added
push
pop, peek
top 3 2
bottom 1
front
back 1 2 3
remove, peek
add
queue
stack

3. Collection efficiency
Complexity class of various operations on collections:
Could we build lists differently to optimize other operations?
Method
ArrayList
Stack
Queue
add (or push)
O(1)
add(index, value) -
indexOf
get
remove
set
size
Method
ArrayList
Stack
Queue
add (or push)
O(1)
add(index, value)
O(N) -
indexOf
get
remove
set
size

4. Memory for a List Array (contiguous in memory) Spread in memory 42 -317 9 42 9 -3 17

6. A list node class Each list node object stores:
public class ListNode {
int data;
ListNode next; }
Each list node object stores:
one piece of integer data
a reference to another list node
ListNodes can be "linked" into chains to store a list of values:
data
next 42
data
next -3
data
next 17
data
next 9
end

7. Linked node problem 1 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list
data
next 10
data
next 20
data
next 30
list

8. Reassigning references
when you say:
a.next = b.next;
you are saying:
"Make variable a.next refer to the same value as b.next."
Or, "Make a.next point to the same place that b.next points."
data
next 10
data
next 20 a
data
next 30
data
next 40 b

9. Linked node problem 2 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list
data
next 30
data
next 10
data
next 20
list

10. Linked node problem 3 What set of statements turns this picture:
Into this?
data
next 10
data
next 20
list1
data
next 30
data
next 40
list2
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2

11. Linked node problem 3 How many ListNode variables?
Which variables change? 2 3
data
next 10
data
next 20
list1 1 5 6
data
next 30
data
next 40
list2 4 3 5
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2 4

12. Linked node problem 3 How many ListNode variables?
Which variables change? 2 3
data
next 10
data
next 20
list1
list1.next.next = list2 1 5 6
data
next 30
data
next 40
list2 4 3 5
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2 4

13. Linked node problem 3 How many ListNode variables?
Which variables change? 2 3
data
next 10
data
next 20
list1
list1.next.next = list2 1
list2 = list2.next 5 6
data
next 30
data
next 40
list2 4 3 5
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2 4

14. Linked node problem 3 How many ListNode variables?
Which variables change? 2 3
data
next 10
data
next 20
list1
list1.next.next = list2 1
list2 = list2.next
list1.next.next.next = null 5 6
data
next 30
data
next 40
list2 4 3 5
data
next 10
data
next 20
data
next 30
list1
data
next 40
list2 4

15. References vs. objects variable = value; For the list at right:
a variable (left side of = ) is an arrow (the base of an arrow)
a value (right side of = ) is an object (a box; what an arrow points at)
For the list at right:
a.next = value; means to adjust where points
variable = a.next; means to make variable point at 2
data
next 10
data
next 20 a 1 1 2

16. data
next 10
data
next 20
data
next 30
list1
current

17. Linked List vs. Array Similar to array code: Print list values:
int[] a = ...;
int i = 0;
while (i < a.length) {
System.out.println(a[i]);
i++; }
Print list values:
ListNode list= ...;
ListNode current = list;
while (current != null) {
System.out.println(current.data);
current = current.next; }
Description
Array Code
Linked List Code
Go to front of list
int i = 0;
ListNode current = list;
Test for more elements
i < size
current != null
Current value
elementData[i]
current.data
Go to next element
i++;
current = current.next;

18. Linked node problem 4 What set of statements turns this picture:
Into this?
data
next 10
data
next
990
list
...
data
next 10
data
next
990
data
next
1000
list
...