130 lines
3.1 KiB
Java
130 lines
3.1 KiB
Java
package DataStructures.Queues;
|
|
|
|
/**
|
|
* This class implements a PriorityQueue.
|
|
* <p>
|
|
* A priority queue adds elements into positions based on their priority.
|
|
* So the most important elements are placed at the front/on the top.
|
|
* In this example I give numbers that are bigger, a higher priority.
|
|
* Queues in theory have no fixed size but when using an array
|
|
* implementation it does.
|
|
*/
|
|
class PriorityQueue {
|
|
/**
|
|
* The max size of the queue
|
|
*/
|
|
private int maxSize;
|
|
/**
|
|
* The array for the queue
|
|
*/
|
|
private int[] queueArray;
|
|
/**
|
|
* How many items are in the queue
|
|
*/
|
|
private int nItems;
|
|
|
|
/**
|
|
* Constructor
|
|
*
|
|
* @param size Size of the queue
|
|
*/
|
|
public PriorityQueue(int size) {
|
|
maxSize = size;
|
|
queueArray = new int[size];
|
|
nItems = 0;
|
|
}
|
|
|
|
/**
|
|
* Inserts an element in it's appropriate place
|
|
*
|
|
* @param value Value to be inserted
|
|
*/
|
|
public void insert(int value) {
|
|
if (isFull()) {
|
|
throw new RuntimeException("Queue is full");
|
|
}
|
|
if (nItems == 0) {
|
|
queueArray[0] = value;
|
|
} else {
|
|
int j = nItems;
|
|
while (j > 0 && queueArray[j - 1] > value) {
|
|
queueArray[j] = queueArray[j - 1]; // Shifts every element up to make room for insertion
|
|
j--;
|
|
}
|
|
queueArray[j] = value; // Once the correct position is found the value is inserted
|
|
}
|
|
nItems++;
|
|
}
|
|
|
|
/**
|
|
* Remove the element from the front of the queue
|
|
*
|
|
* @return The element removed
|
|
*/
|
|
public int remove() {
|
|
return queueArray[--nItems];
|
|
}
|
|
|
|
/**
|
|
* Checks what's at the front of the queue
|
|
*
|
|
* @return element at the front of the queue
|
|
*/
|
|
public int peek() {
|
|
return queueArray[nItems - 1];
|
|
}
|
|
|
|
/**
|
|
* Returns true if the queue is empty
|
|
*
|
|
* @return true if the queue is empty
|
|
*/
|
|
public boolean isEmpty() {
|
|
return (nItems == 0);
|
|
}
|
|
|
|
/**
|
|
* Returns true if the queue is full
|
|
*
|
|
* @return true if the queue is full
|
|
*/
|
|
public boolean isFull() {
|
|
return (nItems == maxSize);
|
|
}
|
|
|
|
/**
|
|
* Returns the number of elements in the queue
|
|
*
|
|
* @return number of elements in the queue
|
|
*/
|
|
public int getSize() {
|
|
return nItems;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* This class implements the PriorityQueue class above.
|
|
*
|
|
* @author Unknown
|
|
*/
|
|
public class PriorityQueues {
|
|
/**
|
|
* Main method
|
|
*
|
|
* @param args Command Line Arguments
|
|
*/
|
|
public static void main(String[] args) {
|
|
PriorityQueue myQueue = new PriorityQueue(4);
|
|
myQueue.insert(10);
|
|
myQueue.insert(2);
|
|
myQueue.insert(5);
|
|
myQueue.insert(3);
|
|
// [2, 3, 5, 10] Here higher numbers have higher priority, so they are on the top
|
|
|
|
for (int i = 3; i >= 0; i--)
|
|
System.out.print(myQueue.remove() + " "); // will print the queue in reverse order [10, 5, 3, 2]
|
|
|
|
// As you can see, a Priority Queue can be used as a sorting algotithm
|
|
}
|
|
}
|