// Checks if a DoublyLinkedList is a palindrome
// which includes an empty or single-element DoublyLinkedList.
// Running time: O(n).
public bool IsPalindrome()
{
// Initializes the current node and a stack.
DLLNode current = first;
Stack <int> valueStack = new Stack <int>();
// Transfers all values in a DoublyLinkedList to a stack.
while (current != null)
{
valueStack.Push(current.Value);
current = current.next;
}
// Initalizes the current node.
current = first;
// Checks if a DoublyLinkedList is a palindrome.
for (int i = 0; i < valueStack.Count / 2; i++)
{
if (current.Value != valueStack.Pop())
{
return(false);
}
current = current.next;
}
return(true);
}
// Inserts a node with a specified value in a sorted DoublyLinkedList.
// Running time: O(n).
public void Insert(int val)
{
// If a DoublyLinkedList is empty
// or a specified value is smaller than the value of the first node,
// adds a new node at the beginning of a DoublyLinkedList.
if (IsEmpty() || first.Value > val)
{
AddFirst(val);
}
else
{
// Creates a new node with the specified value.
DLLNode newNode = new DLLNode(val);
// Sets the current node as the last node of a DoublyLinkedList
// or right before a node that contains a value
// that is greater than or equal to a specified value.
DLLNode curr = first;
while (curr.next != null && curr.next.Value < val)
{
curr = curr.next;
}
// Inserts a new node between the current and the next node.
newNode.previous = curr;
newNode.next = curr.next;
curr.next = newNode;
if (newNode.next != null)
{
newNode.next.previous = newNode;
}
}
}
// Remove duplicate values of a DoublyLinkedList.
// Running time: O(n^2).
public void RemoveDuplicates()
{
// Initializes the current node.
DLLNode current = first;
// Initializes the node that will be right before
// the node current node will be compared to.
DLLNode beforeComparer = first;
// Removes duplicate values.
while (current != null && beforeComparer.next != null)
{
while (beforeComparer.next != null)
{
if (current.Value == beforeComparer.next.Value)
{
beforeComparer.next = beforeComparer.next.next;
}
else
{
beforeComparer = beforeComparer.next;
}
}
// Sets the current node as the next node in a DoublyLinkedList.
current = current.next;
// Sets the node that will be right before
// the node current node will be compared to.
beforeComparer = current;
}
}
// Delete a node of a DoublyLinkedList that contains a specified value.
// Running time: O(n).
public void Delete(int val)
{
if (IsEmpty())
{
throw new Exception("You can't delete an element from an empty list.");
}
else if (val == first.Value)
{
RemoveFirst();
}
else
{
// Sets the current node as the node
// right before a node that contains a specified value
// or the last node of a DoublyLinkedList.
DLLNode curr = first;
while (curr.next != null && curr.next.Value != val)
{
curr = curr.next;
}
// If a node that contains a specified value does not exist.
if (curr.next == null)
{
throw new Exception("A node that contains a specified value does not exist.");
}
// If a node that contains a spcified value exists, deletes the node.
else
{
curr.next = curr.next.next;
curr.next.next.previous = curr;
}
}
}
// Removes the last node of a DoublyLinkedList.
// Running time: O(n).
public void RemoveLast()
{
if (IsEmpty())
{
throw new IndexOutOfRangeException("You can't remove the last element from an emtpy list.");
}
// If there is only one element in a DoublyLinkedList.
else if (first.next == null)
{
// Removes the only element from a DoublyLinkedList.
first = null;
}
else
{
// Sets the current node as the node before the last node.
DLLNode current = first;
while (current.next.next != null)
{
current = current.next;
}
// Removes the last node of a DoublyLinkedList.
current.next = null;
}
}
// Creates a new node with a value
// and adds it at the beginning of a DoublyLinkedList.
// Running time: O(1).
public void AddFirst(int value)
{
DLLNode newNode = new DLLNode(value);
if (!this.IsEmpty())
{
first.previous = newNode;
newNode.next = first;
}
first = newNode;
}
// Removes the first node of a DoublyLinkedList.
// Running time: O(1).
public void RemoveFirst()
{
if (IsEmpty())
{
throw new IndexOutOfRangeException("You can't remove the first element from an empty list.");
}
else
{
first = first.next;
first.previous = null;
}
}
// Prints all values of nodes in a DoublyLinkedList.
// Running time: O(n).
public void PrintList()
{
if (IsEmpty())
{
Console.Write("The DoublyLinkedList is empty.");
}
else
{
// Prints the entire DoublyLinkedList.
DLLNode current = first;
while (current != null)
{
Console.Write(current.Value + " ");
current = current.next;
}
}
Console.WriteLine();
}
// Creates a new node and adds it at the end of a DoublyLinkedList.
// Running time: O(n).
public void AddLast(int val)
{
// If a DoublyLinked is empty,
// adds it at the beginning of a DoublyLinkedList.
if (IsEmpty())
{
AddFirst(val);
}
else
{
DLLNode newNode = new DLLNode(val);
// Sets "current" node to the last node of a DoublyLinkedList.
DLLNode current = first;
while (current.next != null)
{
current = current.next;
}
current.next = newNode;
newNode.previous = current;
}
}
// Prints all values of nodes in a DoublyLinkedList in a reverse way.
// Running time: O(n).
public void printReverse()
{
if (IsEmpty())
{
Console.Write("The DoublyLinkedList is empty.");
}
else
{
// Sets the current node as the last node in a DoublyLinkedList.
DLLNode current = first;
while (current.next != null)
{
current = current.next;
}
// Prints the entire DoublyLinkedList.
while (current != null)
{
Console.Write(current.Value + " ");
current = current.previous;
}
}
}
// A DoublyLinkedList constructor.
public DoublyLinkedList()
{
first = null;
}
// Constructor.
public DLLNode(int value)
{
Value = value;
previous = null;
next = null;
}
