DSA Reviewer:
Heaps ~ is a complete binary tree where the value of each of each parent node is either
higher or lower than the value of its child nodes.
Two types of Heaps:
•
•
Min Heap – The value of each parent node is less than or equal to the values of
its child nodes.
Max Heap – The value of each parent node is greater than or equal to the values
of its child nodes.
Notes: Heaps can be
Java using ArrayList from the java.util package.
implemented in
Example: ArrayList<Integer> heap = new ArrayList<>();
addAll() method - used to create heap with initial values.
Example (based on the given min heap above):
ArrayList<Integer> minHeap = new ArrayList<>();
Collections.addAll(minHeap, 1, 7, 6, 8, 9);
Explanation: The root node is always at index 0. Its child nodes are always at index 1
and index 2. Indices 3, 4, 5, and 6 can store the child nodes of those two (2) nodes and
so on.
get() method – used to determine the root node.
Example: System.out.print(minHeap.get(0));
Priority Queues - is a special type of queue where elements are processed based on
their order (natural or custom).
Notes: Can be implemented in Java using the PriorityQueue class from the java.util
package.
Example: PriorityQueue<Integer> printer = new PriorityQueue<>();
add() or offer() – use to enqueue.
poll() or remove() – used to dequeue.
Notes: The dequeue operation starts with the minimum element.
Comparator - is used to create a specific ordering for a collection of objects.
Notes: To create a comparator in Java, use the Comparator interface and its methods
such as comparing(), comparingInt(), and compare().
Sets - is a collection of elements where each element is unique.
Common set operations:
Union: The union of two (2) sets (A ∪ B) is the set that contains all the elements in
either set.
Example: {1, 3, 5, 7} ∪ {2, 3, 4, 5} = {1, 2, 3, 4, 5, 7}
Intersection: The intersection of two (2) sets (A ∩ B) is the set that contains only the
elements common to both sets.
Example: {1, 3, 5, 7} ∩ {2, 3, 4, 5} = {3, 5}
Difference: The difference of sets A and B (A – B) is the set that contains the elements
that are in A but not in B.
Example: {1, 3, 5, 7} – {2, 3, 4, 5} = {1, 7}
Subset: Set A is a subset of set B (A ⊂ B) if every element of set A is also an element of
set B.
Example: {1, 3, 5, 7} ⊂ {1, 2, 3, 4, 5, 7} = true
Three (3) general-purpose set implementations. All are included in the java.util package.
HashSet – This stores its elements in a hash table without a guaranteed order upon
iteration. This is the best-performing implementation.
TreeSet – This stores its elements in a special type of tree where elements are sorted
(natural or custom) during iteration.
LinkedHashSet – This stores its elements in a hash table with a linked list running
through it. The order of the elements during the iteration is the same as the order they
were inserted into the set.
Maps - is a set of ordered pairs where elements are known as keys (identifiers) and
values (content).
Three (3) general-purpose map implementations.
TreeMap - is a data structure for storing and organizing objects according to key-value
pairs.
LinkedHashMap - is an extension of the HashMap class and it implements the Map
interface.
HashMap - stores the data in (Key, Value) pairs, and you can access them by an index
of another type (e.g. an Integer).
Notes: A map cannot contain duplicate keys. Each key can map to only one (1)
value.
Example:
Key Values: Map of first Names
K
Kae
M1
Mark
M2
Mairo
Dictionaries – known as maps in python.
Note: To create a dictionary, use the colon symbol (:) for key-value pairs within
the curly braces.