Post on 02-Jun-2020
CS106X –Programming
Abstractions in C++
Dr. Cynthia Bailey Lee
Today’s Topics:
1. Linked structures
2. (if we have time) Heaps
2
What does this code do?front->next->next = new ListNode;
front->next->next->data = 40;
Before:
A. After:
B. After:
C. Using “next” that is NULL gives error
D. Other/none/more than one
data next
10
data next
20 NULLfront
data next
10
data next
40front
data next
20 NULL
data next
10
data next
20front
data next
40 NULL
Need to add this: front->next->next->next = NULL;
Lets clean this code up a bit Add LinkedNode constructors to help initialize new nodes
struct LinkedNode {
int data;
LinkedNode * next;
LinkedNode() {
data = 0;
next = NULL;
}
LinkedNode(int d, LinkedNode* n) {
data = d;
next = n;
}
};
Lets clean this code up a bit Make a LinkedList class (separate from LinkedNode) to
encapsulate list operations like generalized insert() and
remove() to insert and remove at given indicies
class LinkedList {
public:
LinkedList();
~LinkedList();
void insert(int data, unsigned int index);
void remove(unsigned int index);
int get(unsigned int index);
private:
LinkedNode * head;
}
Add to front
head
head
NULL
NULL
Add to the end
Special cases of add
head
NULL
Add to the middle
NULL
Lets clean this code up a bit
//PREcondition: assume index is valid
void LinkedList::insert(int data, unsigned int index){
if (index == 0){
head = new LinkedNode(data, head);
return;
}
LinkedNode * current = head;
unsigned int i=0;
for (int i=0; i<index-1; i++){//assumes index valid
current = current->next;
}
current->next = new LinkedNode(data, current->next);
}
Remove from the front
NULL
head
Special cases of remove
Remove in the middle
NULL
head
NULL
head Remove from the end
NULL
Suppose we have a pointer (current) to the node
containing the item to be removed.
What additional information do we need to
successfully remove the node?
A) Nothing additional.
B) A pointer to the node immediately prior to the to-be-deleted
node.
C) A pointer to the node immediately after the to-be-deleted
node.
D) Both B and C.
NULL
head current (we want to remove this one)
Remove
Suppose we have a pointer (current) to the node
containing the item to be removed.
NULL
head current (we want to remove this one)
NINJA CODER EDITION
Remove (bug)
void LinkedList::remove(unsigned int index){
if (index == 0){
head = head->next;
return;
}
LinkedNode * current = head;
unsigned int i=0;
for (i=0; i<index-1; i++){
current = current->next;
}
current->next = current->next->next;
}
Classic interview questions
How can you remove a node of a linked list
given a pointer to the node itself (and not the
previous one)?
Write code to reverse a linked list in place.
How can you detect if there is a loop in a
linked list (i.e. list is improperly formed), using
constant space (O(1) amount of space)?
Priority QueueEmergency Department waiting room operates as a
priority queue: patients are sorted according to
seriousness, NOT how long they have waited.
Some priority queue
implementation options
Unsorted linked list
Insert new element in front
Remove by searching list for highest-
priority item
Sorted linked list
Always insert new elements where they
go in priority-sorted order
Remove from front
Unsorted linked list
Add is FAST
Just throw it in the list
at the front
O(1)
Remove/peek is SLOW
Hard to find item the
highest priority item—
could be anywhere
O(N)
Priority queue implementations
This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license. Keyah Cheatum http://commons.wikimedia.org/wiki/File:Messy_Room.JPG
Sorted linked list
Add is SLOW
Need to step through the list to find where item goes in priority-sorted order
O(N)
Remove/peek is FAST
Easy to find item you are looking for (first in list)
O(1)
Priority queue implementations
We want the best of both
Fast add AND fast remove/peek
We will investigate trees as a way to get
the best of both worlds
Priority queue implementations