Decode the Coding Interview in Scala: Real-World Examples/

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Feature #15: Queue Reconstruction by Priority

Feature #15: Queue Reconstruction by Priority

Implement the "Queue Reconstruction by Priority" feature for our "Operating System" project.

We'll cover the following...

Description

A process queue contains the process priority. It also contains the number of processes ahead of a process in the queue that has a priority not less than its own. Suppose that the OS crashed and now we only have an array of processes, with each process at a random position in the array. In this feature, we’ll reconstruct the process queue from the information available to us.

Each element in the 2D array consists of a process’s priority and the number of processes with a higher or equal priority that are ahead of it in the queue. An entry [pi, ki] represents that a process with priority pi has ki other processes, with a priority of at least pi, ahead of it in the queue.

Our task is to reconstruct and return the process queue.

Let’s look at a few examples of this:

Solution

A process with a lower priority does not affect the placement of k processes with a higher priority. So, we will first insert the processes with a higher priority, into the output array. We will start by sorting the input array in descending order of process priority, and then in ascending order of the k-value. We will:

  • Sort the processes by priority, in a descending order.
  • Sort the processes with the same priority in ascending order of k.

We will pick elements from the sorted array, starting at index 0. If the element picked is [pi, ki], it will be inserted at index k in the output array. The following slides demonstrate this procedure:

Let’s take a look at an example of this:

import scala.collection.mutable.ListBuffer
import scala.runtime.ScalaRunTime._
object Main extends App{
  def reconstructQueue(people: Array[Array[Int]]): Array[Array[Int]] = {
    val sorted = people.sortBy(arr => (arr.head, arr.last))(Ordering.Tuple2(Ordering.Int.reverse, Ordering.Int))
    val res = ListBuffer[Array[Int]]()
    sorted.foreach(p => res.insert(p.last, p))
    res.toArray
  }
  var p = Array(Array(7,0), Array(4,4), Array(7,1), Array(5,0), Array(6,1), Array(5,2))
  var output = reconstructQueue(p)
  print("[")
  for(list <- output)
    print("[" + list.mkString(", ") + "]")
  println("]")
}
Queue Reconstruction by Priority
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