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PriorityQueue.java
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PriorityQueue.java
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/*
* Title: Priority Queue
* Author: Sam Gavis-Hughson
* Date: 7/24/2017
*
* Implement a priority queue
*
* Execution: javac PriorityQueue.java && java PriorityQueue
*
* For more details, check out http://www.byte-by-byte.com/priorityqueue/
*/
// Implements a priority queue using a max heap. The heap is of fixed size and
// represented using an array.
import java.util.Arrays;
public class PriorityQueue {
int[] heap;
int size;
// Constructor initializes instance variables
public PriorityQueue(int maxSize) {
heap = new int[maxSize];
size = 0;
}
// Push an element on to the queue
public void push(int val) {
// If the queue is full then throw an exception
if (size == heap.length) throw new IllegalStateException();
// Put the value in the next available space in the queue
int pos = size;
heap[pos] = val;
// While val is bigger than its parent, swap it with its parent
while (pos > 0) {
// Get the parent and compare the values
int parent = (pos+1) / 2 - 1;
if (heap[parent] >= heap[pos]) break;
swapIndices(parent, pos);
pos = parent;
}
// We added an element so increment the size
size++;
}
// Pop the max element from the queue
public int pop() {
// If the queue is empty, throw an exception
if (size == 0) throw new IllegalStateException ();
// The top of the heap is the first item in the array, so save it off
// to the side so we don't lose it
int toReturn = heap[0];
// Move the bottom item in the heap to the first position. We don't need
// to remove it from the array because we have the size variable
heap[0] = heap[size - 1];
size--;
// Bubble down the top element to the right spot
int pos = 0;
// We're going to be swapping with the children and any pos >= size / 2
// doesn't have any children
while (pos < size / 2) {
int leftChild = pos * 2 + 1;
int rightChild = leftChild + 1;
// If the right child exists and is greater than the left child,
// compare it to the current position
if (rightChild < size && heap[leftChild] < heap[rightChild]) {
// Only swap if the value is less than the child
if (heap[pos] >= heap[rightChild]) break;
swapIndices(pos, rightChild);
pos = rightChild;
} else {
// Do the same comparison with the left child
if (heap[pos] >= heap[leftChild]) break;
swapIndices(pos, leftChild);
pos = leftChild;
}
}
return toReturn;
}
// Swap the values at the indices
private void swapIndices(int i, int j) {
int temp = heap[i];
heap[i] = heap[j];
heap[j] = temp;
}
public static void main(String[] args) {
tester(new int[]{}, "Empty");
tester(new int[]{1, 2, 3, 4, 5, 6, 7, 8}, "Ascending order");
tester(new int[]{8, 7, 6, 5, 4, 3, 2, 1}, "Descending order");
tester(new int[]{1, 6, 3, 4, 5, 2, 8, 7}, "Mixed");
System.out.println("Passed all test cases");
}
private static void tester(int[] input, String name) {
PriorityQueue p = new PriorityQueue(input.length);
for (int i : input) {
p.push(i);
}
Arrays.sort(input);
for (int i = input.length - 1; i >= 0; i--) {
assert input[i] == p.pop():
name;
}
}
}