14.
Chapter 14 Solutions
Written by Jonathan Sande & Vincent Ngo
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Solution to Challenge 1
Start with the following Person type:
class Person extends Comparable<Person> {
Person({
required this.name,
required this.age,
required this.isMilitary,
});
final String name;
final int age;
final bool isMilitary;
@override
int compareTo(other) => throw UnimplementedError();
}
Since a priority queue needs to compare elements, Person also needs to be Comparable.
Given a list of people on the waitlist, you would like to prioritize the people in the following order:
- Military background
- Seniority, by age
The key to solving this problem is to finish implementing the compareTo method in Person so that you can use a priority queue to tell you the order of people on the waitlist. Replace compareTo with the following code:
@override
int compareTo(other) {
if (isMilitary == other.isMilitary) {
return age.compareTo(other.age);
}
return isMilitary ? 1 : -1;
}
If two people have the same military background, then age is used to see who has the highest priority. But if the military background is different, then the one having a military background is prioritized.
Before you test your implementation out, override toString so that Person is printable:
@override
String toString() {
final military = (isMilitary) ? ', (military)' : '';
return '$name, age $age$military';
}
Import the PriorityQueue that you made earlier in the chapter if you haven’t already. Then run the following example in main:
final p1 = Person(name: 'Josh', age: 21, isMilitary: true);
final p2 = Person(name: 'Jake', age: 22, isMilitary: true);
final p3 = Person(name: 'Clay', age: 28, isMilitary: false);
final p4 = Person(name: 'Cindy', age: 28, isMilitary: false);
final p5 = Person(name: 'Sabrina', age: 30, isMilitary: false);
final waitlist = [p1, p2, p3, p4, p5];
var priorityQueue = PriorityQueue(elements: waitlist);
while (!priorityQueue.isEmpty) {
print(priorityQueue.dequeue());
}
You should see the output below:
Jake, age 22, (military)
Josh, age 21, (military)
Sabrina, age 30
Clay, age 28
Cindy, age 28
Solution to Challenge 2
To make a list-based priority queue, all you have to do is implement the Queue interface. Instead of using a heap, though, you use a list data structure.
abstract class Queue<E> {
bool enqueue(E element);
E? dequeue();
bool get isEmpty;
E? get peek;
}
Getting Started
First, add the following code to a project that contains the Queue interface:
enum Priority { max, min }
class PriorityQueueList<E extends Comparable<dynamic>> implements Queue<E> {
PriorityQueueList({List<E>? elements, Priority priority = Priority.max}) {
_priority = priority;
_elements = elements ?? [];
}
late List<E> _elements;
late Priority _priority;
// more to come
}
Which Is the High Priority End?
At this point you need to make a decision. You can either put the high priority elements at the start of the list or at the end of the list.
Sorting an Initial List
Replace the PriorityQueueList constructor with the following code:
PriorityQueueList({List<E>? elements, Priority priority = Priority.max}) {
_priority = priority;
_elements = elements ?? [];
_elements.sort((a, b) => _compareByPriority(a, b));
}
int _compareByPriority(E a, E b) {
if (_priority == Priority.max) {
return a.compareTo(b);
}
return b.compareTo(a);
}
Implementing isEmpty and peek
Add the following methods to begin implementing the Queue interface:
@override
bool get isEmpty => _elements.isEmpty;
@override
E? get peek => (isEmpty) ? null : _elements.last;
Implementing enqueue
Next, add the enqueue method:
@override
bool enqueue(E element) {
// 1
for (int i = 0; i < _elements.length; i++) {
// 2
if (_compareByPriority(element, _elements[i]) < 0) {
// 3
_elements.insert(i, element);
return true;
}
}
// 4
_elements.add(element);
return true;
}
Implementing dequeue
Next add the dequeue method:
@override
E? dequeue() => isEmpty ? null : _elements.removeLast();
Making the Queue Printable
Finally, override toString so that you can print your priority queue in a friendly format:
@override
String toString() => _elements.toString();
Testing it Out
To test out the priority queue, run the following in main:
final priorityQueue = PriorityQueueList(
elements: [1, 12, 3, 4, 1, 6, 8, 7],
);
print(priorityQueue);
priorityQueue.enqueue(5);
priorityQueue.enqueue(0);
priorityQueue.enqueue(10);
print(priorityQueue);
while (!priorityQueue.isEmpty) {
print(priorityQueue.dequeue());
}
[1, 1, 3, 4, 6, 7, 8, 12]
[0, 1, 1, 3, 4, 5, 6, 7, 8, 10, 12]
12
10
8
7
6
5
4
3
1
1
0