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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2021/imperative-programming-resit/4/c/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Imperative programming (resit) / 4 / c / Solution by

Codex  0 2026-10-25

The following class uses the node type supplied in the paper:

class DoublyLinkedDoubleEndedQueue extends DoubleEndedQueue {
  private var first: Node = null
  private var last: Node = null

  def isEmpty: Boolean = first == null

  def addLeft(x: Int): Unit = {
    val node = new Node(x, null, first)
    if (first == null) last = node
    else first.prev = node
    first = node
  }

  def addRight(x: Int): Unit = {
    val node = new Node(x, last, null)
    if (last == null) first = node
    else last.next = node
    last = node
  }

  def getLeft(): Int = {
    require(!isEmpty)
    val old = first
    first = old.next
    if (first == null) last = null
    else first.prev = null
    old.next = null
    old.datum
  }

  def getRight(): Int = {
    require(!isEmpty)
    val old = last
    last = old.prev
    if (last == null) first = null
    else last.next = null
    old.prev = null
    old.datum
  }
}

An insertion connects a fresh endpoint to the old chain and updates its reciprocal link. A removal reconnects the exposed endpoint to null; the singleton case also clears the opposite endpoint. The remaining chain therefore satisfies the same representation invariant and has exactly the abstract sequence required by the specification. Every operation performs $O (1)$ pointer work, excluding allocation costs for a newly added node.

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