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.
To run the tests, run the command dotnet test
from within the exercise directory.
Initially, only the first test will be enabled. This is to encourage you to solve the exercise one step at a time.
Once you get the first test passing, remove the Skip
property from the next test and work on getting that test passing.
Once none of the tests are skipped and they are all passing, you can submit your solution
using exercism submit LinkedList.cs
For more detailed information about the C# track, including how to get help if you're having trouble, please visit the exercism.io C# language page.
Classic computer science topic
using Xunit;
public class DequeTests
{
[Fact]
public void Push_and_pop_are_first_in_last_out_order()
{
var deque = new Deque<int>();
deque.Push(10);
deque.Push(20);
Assert.Equal(20, deque.Pop());
Assert.Equal(10, deque.Pop());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void Push_and_shift_are_first_in_first_out_order()
{
var deque = new Deque<int>();
deque.Push(10);
deque.Push(20);
Assert.Equal(10, deque.Shift());
Assert.Equal(20, deque.Shift());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void Unshift_and_shift_are_last_in_first_out_order()
{
var deque = new Deque<int>();
deque.Unshift(10);
deque.Unshift(20);
Assert.Equal(20, deque.Shift());
Assert.Equal(10, deque.Shift());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void Unshift_and_pop_are_last_in_last_out_order()
{
var deque = new Deque<int>();
deque.Unshift(10);
deque.Unshift(20);
Assert.Equal(10, deque.Pop());
Assert.Equal(20, deque.Pop());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void Push_and_pop_can_handle_multiple_values()
{
var deque = new Deque<int>();
deque.Push(10);
deque.Push(20);
deque.Push(30);
Assert.Equal(30, deque.Pop());
Assert.Equal(20, deque.Pop());
Assert.Equal(10, deque.Pop());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void Unshift_and_shift_can_handle_multiple_values()
{
var deque = new Deque<int>();
deque.Unshift(10);
deque.Unshift(20);
deque.Unshift(30);
Assert.Equal(30, deque.Shift());
Assert.Equal(20, deque.Shift());
Assert.Equal(10, deque.Shift());
}
[Fact(Skip = "Remove this Skip property to run this test")]
public void All_methods_of_manipulating_the_deque_can_be_used_together()
{
var deque = new Deque<int>();
deque.Push(10);
deque.Push(20);
Assert.Equal(20, deque.Pop());
deque.Push(30);
Assert.Equal(10, deque.Shift());
deque.Unshift(40);
deque.Push(50);
Assert.Equal(40, deque.Shift());
Assert.Equal(50, deque.Pop());
Assert.Equal(30, deque.Shift());
}
}
using System;
public class Deque<T>
{
private class Node
{
public Node Previous = null;
public Node Next = null;
public T Value;
}
private Node Head = new Node();
private Node Tail = new Node();
private int count = 0;
private bool IsEmpty => count == 0;
public Deque()
{
Head.Previous = Tail;
Tail.Next = Head;
}
public void Push(T value) => Insert(new Node { Value = value }, Head.Previous, Head);
public T Pop()
{
if (IsEmpty)
{
throw new InvalidOperationException("Cannot pop an empty collection.");
}
return Remove(Head.Previous).Value;
}
public void Unshift(T value) => Insert(new Node { Value = value }, Tail, Tail.Next);
public T Shift()
{
if (IsEmpty)
{
throw new InvalidOperationException("Cannot shift an empty collection.");
}
return Remove(Tail.Next).Value;
}
private void Insert(Node node, Node previous, Node next)
{
node.Next = next;
node.Previous = previous;
previous.Next = node;
next.Previous = node;
++count;
}
private Node Remove(Node node)
{
node.Previous.Next = node.Next;
node.Next.Previous = node.Previous;
node.Next = null;
node.Previous = null;
--count;
return node;
}
}
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