CS106X –
Programming
Abstractions
Dr. Cynthia Bailey Lee
2
Today’s Topics
1.
TreeMap


2.
Tree traversals
Abstract syntax tree (calculator)
Hashmap (if we have time)

A quick look inside hash functions
Tree traversals
What does this print?
void traverse(Node *node) {
if (node != NULL) {
cout << node->key << " ";
traverse(node->left);
traverse(node->right);
}
}
A.
B.
C.
D.
E.
ABCDEF
ABDECF
DBEFCA
DEBFCA
Other/none/more
A
B
D
C
E
F
What does this print?
void traverse(Node *node) {
if (node != NULL) {
traverse(node->left);
cout << node->key << " ";
traverse(node->right);
}
}
A.
B.
C.
D.
E.
ABCDEF
ABDECF
DBEFCA
DEBFCA
Other/none/more
A
B
D
C
E
F
We can play the same game with non-alpha
characters as keys: What does this print?
void traverse(Node *node) {
if (node != NULL) {
traverse(node->left);
traverse(node->right);
cout << node->key << " ";
}
}
A.
B.
C.
D.
E.
*+/3482
*+34/2
3+4*8/2
34+82/*
Other/none/more
*
+
3
/
4
8
2
Remember our old friend:
How can we get this to print our ABCs
in order?
void traverse(Node *node) {
if (node != NULL) {
cout << node->key << " ";
traverse(node->left);
traverse(node->right);
}
D
}
A.
B.
C.
D.
A
B
C
E
F
You can’t—we already tried moving the cout to all
3 possible places.
You can but you use a stack instead of recursion.
You can but you use a queue instead of recursion.
Other/none/more
That prints the ABC nodes in order, but notice
that our ABC tree isn’t a BST. How do we print BST
nodes in order?
void traverse(Node *node) {
if (node != NULL) {
traverse(node->left);
traverse(node->right);
}
}
A.
B.
C.
D.
E.
Use the BFS (queue instead of recursion).
Use the DFS and put cout first.
Use the DFS and put cout between.
Use the DFS and put cout last.
Other/none/more
Hashmap
A peek inside the Stanford library implementation
Recall this slide from Monday. We will compare what we said
here with what is done in Stanford library:
One really simple approach



Hash table (array) size is some prime number
Keep the array size big enough that at most 70%
of the slots are filled
Somehow convert your data to an int and then
int hash(int input){
return input % size;
}


Can we still get super unlucky? YES
Hashmap analysis of O(1) is “average case”

For a really nicely designed hash: 99.9% of the time