Priority Queue Algorithm
The Priority Queue Algorithm is a data structure that allows efficient access to the highest-priority element among a set of elements. It is an abstract data type that supports two primary operations: inserting an element with an associated priority, and removing the element with the highest priority. The priority queue can be implemented using various data structures such as arrays, linked lists, or heaps. Among these, the binary heap, specifically the min-max heap, is the most commonly used data structure for implementing priority queues due to its efficiency in insertion and removal operations.
In a priority queue, elements are ordered based on their priority values. The element with the highest priority is dequeued first, while the element with the lowest priority is dequeued last. This ordering can be achieved through either a min-heap, where the element with the smallest priority value is considered the highest priority, or a max-heap, where the element with the largest priority value is considered the highest priority. Applications of priority queues are found in various domains such as scheduling processes in operating systems, handling network packets by routers, and solving graph-based problems like Dijkstra's shortest path algorithm and Prim's minimum spanning tree algorithm.
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package com.algorithmexamples.datastructures
import com.algorithmexamples.sorts.exch
import java.util.*
import kotlin.Comparator
class PriorityQueue<T>(size: Int, val comparator: Comparator<T>? = null) : Collection<T> {
public override var size: Int = 0
private set
private var arr: Array<T?> = Array<Comparable<T>?>(size, { null }) as Array<T?>
public fun add(element: T) {
if (size + 1 == arr.size) {
resize()
}
arr[++size] = element
swim(size)
}
public fun peek(): T {
if (size == 0) throw NoSuchElementException()
return arr[1]!!
}
public fun poll(): T {
if (size == 0) throw NoSuchElementException()
val res = peek()
arr.exch(1, size--)
sink(1)
arr[size + 1] = null
if ((size > 0) && (size == (arr.size - 1) / 4)) {
resize()
}
return res
}
private fun swim(n: Int) {
Companion.swim(arr, n, comparator)
}
private fun sink(n: Int) {
Companion.sink(arr, n, size, comparator)
}
private fun resize() {
val old = arr
arr = Array<Comparable<T>?>(size * 2, { null }) as Array<T?>
System.arraycopy(old, 0, arr, 0, size + 1)
}
public override fun isEmpty(): Boolean {
return size == 0
}
override fun contains(element: T): Boolean {
for (obj in this) {
if (obj == element) return true
}
return false
}
override fun containsAll(elements: Collection<T>): Boolean {
for (element in elements) {
if (!contains(element)) return false
}
return true
}
override fun iterator(): Iterator<T> {
return arr.copyOfRange(1, size + 1).map { it!! }.iterator()
}
companion object {
private fun<T> greater(arr: Array<T?>, i: Int, j: Int, comparator: Comparator<T>? = null): Boolean {
if (comparator != null) {
return comparator.compare(arr[i], arr[j]) > 0
} else {
val left = arr[i]!! as Comparable<T>
return left > arr[j]!!
}
}
public fun<T> sink(arr: Array<T?>, a: Int, size: Int, comparator: Comparator<T>? = null) {
var k = a
while (2 * k <= size) {
var j = 2 * k
if (j < size && greater(arr, j, j + 1, comparator)) j++
if (!greater(arr, k, j, comparator)) break
arr.exch(k, j)
k = j
}
}
public fun<T> swim(arr: Array<T?>, size: Int, comparator: Comparator<T>? = null) {
var n = size
while (n > 1 && greater(arr, n / 2, n, comparator)) {
arr.exch(n, n / 2)
n /= 2
}
}
}
}
