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SergiiVlasiuk's solution

to Linked List in the Scala Track

Published at Aug 26 2019 · 0 comments
Instructions
Test suite
Solution

Implement a doubly linked list.

Like an array, a linked list is a simple linear data structure. Several common data types can be implemented using linked lists, like queues, stacks, and associative arrays.

A linked list is a collection of data elements called nodes. In a singly linked list each node holds a value and a link to the next node. In a doubly linked list each node also holds a link to the previous node.

You will write an implementation of a doubly linked list. Implement a Node to hold a value and pointers to the next and previous nodes. Then implement a List which holds references to the first and last node and offers an array-like interface for adding and removing items:

  • push (insert value at back);
  • pop (remove value at back);
  • shift (remove value at front).
  • unshift (insert value at front);

To keep your implementation simple, the tests will not cover error conditions. Specifically: pop or shift will never be called on an empty list.

If you want to know more about linked lists, check Wikipedia.

The Scala exercises assume an SBT project scheme. The exercise solution source should be placed within the exercise directory/src/main/scala. The exercise unit tests can be found within the exercise directory/src/test/scala.

To run the tests simply run the command sbt test in the exercise directory.

For more detailed info about the Scala track see the help page.

Source

Classic computer science topic

Submitting Incomplete Solutions

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DequeTest.scala

import org.scalatest.{Matchers, FlatSpec}

/** @version created manually **/
class DequeTest extends FlatSpec with Matchers {

  it should "handle push then pop" in {
    val deque = Deque[Char]()
    deque.push('a')
    deque.push('b')
    deque.pop should be (Some('b'))
    deque.pop should be (Some('a'))
//    linkedList.pop should be (None)
  }

  it should "handle push then shift" in {
    pending
    val deque = Deque[Char]()
    deque.push('a')
    deque.push('b')
    deque.shift should be (Some('a'))
    deque.shift should be (Some('b'))
//    linkedList.shift should be (None)
  }

  it should "handle unshift then shift" in {
    pending
    val deque = Deque[Char]()
    deque.unshift('a')
    deque.unshift('b')
    deque.shift should be (Some('b'))
    deque.shift should be (Some('a'))
  }

  it should "handle unshift then pop" in {
    pending
    val deque = Deque[Char]()
    deque.unshift('a')
    deque.unshift('b')
    deque.pop should be (Some('a'))
    deque.pop should be (Some('b'))
  }

  it should "handle complex interaction" in {
    pending
    val deque = Deque[Int]()
    deque.push(1)
    deque.push(2)
    deque.pop should be (Some(2))
    deque.push(3)
    deque.unshift(4)
    deque.push(5)
    deque.shift should be (Some(4))
    deque.pop should be (Some(5))
    deque.pop should be (Some(3))
  }
}
case class Deque[T]() {
  private var head: Option[Node] = None
  private var last: Option[Node] = None

  case class Node(value: T, var prev: Option[Node], var next: Option[Node])

  def push(value: T) {
    val node = Node(value, last, None)
    last.foreach(_.next = Some(node))
    last = Some(node)
    if (head.isEmpty) head = last
  }

  def unshift(value: T) {
    val node = Node(value, None, head)
    head.foreach(_.prev = Some(node))
    head = Some(node)
    if (last.isEmpty) last = head
  }

  def pop = {
    val value = last.map(_.value)
    last = last.flatMap(_.prev)
    if (last.isEmpty) head = None
    value
  }

  def shift = {
    val value = head.map(_.value)
    head = head.flatMap(_.next)
    if (head.isEmpty) last = None
    value
  }
}

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