DoublyLinkedList.java

package DataStructures.Lists;

/**
 * This class implements a DoublyLinkedList. This is done using the classes LinkedList and Link.
 *
 * <p>A linked list is similar to an array, it holds values. However, links in a linked list do not
 * have indexes. With a linked list you do not need to predetermine it's size as it grows and
 * shrinks as it is edited. This is an example of a double ended, doubly linked list. Each link
 * references the next link and the previous one.
 *
 * @author Unknown
 */
public class DoublyLinkedList {
  /** Head refers to the front of the list */
  private Link head;
  /** Tail refers to the back of the list */
  private Link tail;

  /** Size refers to the number of elements present in the list */
  private int size;

  /** Default Constructor */
  public DoublyLinkedList() {
    head = null;
    tail = null;
    size = 0;
  }

  /**
   * Constructs a list containing the elements of the array
   *
   * @param array the array whose elements are to be placed into this list
   * @throws NullPointerException if the specified collection is null
   */
  public DoublyLinkedList(int[] array) {
    if (array == null) throw new NullPointerException();
    for (int i : array) {
      insertTail(i);
    }
    size = array.length;
  }

  /**
   * Insert an element at the head
   *
   * @param x Element to be inserted
   */
  public void insertHead(int x) {
    Link newLink = new Link(x); // Create a new link with a value attached to it
    if (isEmpty()) // Set the first element added to be the tail
    tail = newLink;
    else head.previous = newLink; // newLink <-- currenthead(head)
    newLink.next = head; // newLink <--> currenthead(head)
    head = newLink; // newLink(head) <--> oldhead
    ++size;
  }

  /**
   * Insert an element at the tail
   *
   * @param x Element to be inserted
   */
  public void insertTail(int x) {
    Link newLink = new Link(x);
    newLink.next = null; // currentTail(tail)     newlink -->
    if (isEmpty()) { // Check if there are no elements in list then it adds first element
      tail = newLink;
      head = tail;
    } else {
      tail.next = newLink; // currentTail(tail) --> newLink -->
      newLink.previous = tail; // currentTail(tail) <--> newLink -->
      tail = newLink; // oldTail <--> newLink(tail) -->
    }
    ++size;
  }

  /**
   * Insert an element at the index
   *
   * @param x Element to be inserted
   * @param index Index(from start) at which the element x to be inserted
   */
  public void insertElementByIndex(int x, int index) {
    if (index > size) throw new IndexOutOfBoundsException("Index: " + index + ", Size: " + size);
    if (index == 0) {
      insertHead(x);
    } else {
      if (index == size) {
        insertTail(x);
      } else {
        Link newLink = new Link(x);
        Link previousLink = head; //
        for (int i = 1; i < index; i++) { // Loop to reach the index
          previousLink = previousLink.next;
        }
        // previousLink is the Link at index - 1 from start
        previousLink.next.previous = newLink;
        newLink.next = previousLink.next;
        newLink.previous = previousLink;
        previousLink.next = newLink;
      }
    }
    ++size;
  }

  /**
   * Delete the element at the head
   *
   * @return The new head
   */
  public Link deleteHead() {
    Link temp = head;
    head = head.next; // oldHead <--> 2ndElement(head)

    if (head == null) {
      tail = null;
    } else {
      head.previous =
          null; // oldHead --> 2ndElement(head) nothing pointing at old head so will be removed
    }
    --size;
    return temp;
  }

  /**
   * Delete the element at the tail
   *
   * @return The new tail
   */
  public Link deleteTail() {
    Link temp = tail;
    tail = tail.previous; // 2ndLast(tail) <--> oldTail --> null

    if (tail == null) {
      head = null;
    } else {
      tail.next = null; // 2ndLast(tail) --> null
    }
    --size;
    return temp;
  }

  /**
   * Delete the element from somewhere in the list
   *
   * @param x element to be deleted
   * @return Link deleted
   */
  public void delete(int x) {
    Link current = head;

    while (current.value != x) { // Find the position to delete
      if (current != tail) {
        current = current.next;
      } else { // If we reach the tail and the element is still not found
        throw new RuntimeException("The element to be deleted does not exist!");
      }
    }

    if (current == head) deleteHead();
    else if (current == tail) deleteTail();
    else { // Before: 1 <--> 2(current) <--> 3
      current.previous.next = current.next; // 1 --> 3
      current.next.previous = current.previous; // 1 <--> 3
    }
    --size;
  }

  /**
   * Inserts element and reorders
   *
   * @param x Element to be added
   */
  public void insertOrdered(int x) {
    Link newLink = new Link(x);
    Link current = head;
    while (current != null && x > current.value) // Find the position to insert
    current = current.next;

    if (current == head) insertHead(x);
    else if (current == null) insertTail(x);
    else { // Before: 1 <--> 2(current) <--> 3
      newLink.previous = current.previous; // 1 <-- newLink
      current.previous.next = newLink; // 1 <--> newLink
      newLink.next = current; // 1 <--> newLink --> 2(current) <--> 3
      current.previous = newLink; // 1 <--> newLink <--> 2(current) <--> 3
    }
    ++size;
  }

  /**
   * Deletes the passed node from the current list
   *
   * @param z Element to be deleted
   */
  public void deleteNode(Link z) {
    if (z.next == null) {
      deleteTail();
    } else if (z == head) {
      deleteHead();
    } else { // before <-- 1 <--> 2(z) <--> 3 -->
      z.previous.next = z.next; // 1 --> 3
      z.next.previous = z.previous; // 1 <--> 3
    }
    --size;
  }

  public static void removeDuplicates(DoublyLinkedList l) {
    Link linkOne = l.head;
    while (linkOne.next != null) { // list is present
      Link linkTwo = linkOne.next; // second link for comparison
      while (linkTwo.next != null) {
        if (linkOne.value == linkTwo.value) // if there are duplicates values then
        l.delete(linkTwo.value); // delete the link
        linkTwo = linkTwo.next; // go to next link
      }
      linkOne = linkOne.next; // go to link link to iterate the whole list again
    }
  }

  /**
   * Reverses the list in place
   *
   * @param l the DoublyLinkedList to reverse
   */
  public void reverse() {
    // Keep references to the head and tail
    Link thisHead = this.head;
    Link thisTail = this.tail;

    // Flip the head and tail references
    this.head = thisTail;
    this.tail = thisHead;

    // While the link we're visiting is not null, flip the
    // next and previous links
    Link nextLink = thisHead;
    while (nextLink != null) {
      Link nextLinkNext = nextLink.next;
      Link nextLinkPrevious = nextLink.previous;
      nextLink.next = nextLinkPrevious;
      nextLink.previous = nextLinkNext;

      // Now, we want to go to the next link
      nextLink = nextLinkNext;
    }
  }

  /** Clears List */
  public void clearList() {
    head = null;
    tail = null;
    size = 0;
  }

  /**
   * Returns true if list is empty
   *
   * @return true if list is empty
   */
  public boolean isEmpty() {
    return (head == null);
  }

  /** Prints contents of the list */
  public void display() { // Prints contents of the list
    Link current = head;
    while (current != null) {
      current.displayLink();
      current = current.next;
    }
    System.out.println();
  }

  /**
   * Prints the contents of the list in reverse order
   */
  public void displayBackwards() {
    Link current = tail;
    while (current != null) {
      current.displayLink();
      current = current.previous;
    }
    System.out.println();
  }
}

/**
 * This class is used to implement the nodes of the linked list.
 *
 * @author Unknown
 */
class Link {
  /** Value of node */
  public int value;
  /** This points to the link in front of the new link */
  public Link next;
  /** This points to the link behind the new link */
  public Link previous;

  /**
   * Constructor
   *
   * @param value Value of node
   */
  public Link(int value) {
    this.value = value;
  }

  /** Displays the node */
  public void displayLink() {
    System.out.print(value + " ");
  }

  /**
   * Main Method
   *
   * @param args Command line arguments
   */
  public static void main(String args[]) {
    DoublyLinkedList myList = new DoublyLinkedList();
    myList.insertHead(13);
    myList.insertHead(7);
    myList.insertHead(10);
    myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->
    myList.displayBackwards();

    myList.insertTail(11);
    myList.display(); // <-- 10(head) <--> 7 <--> 13 <--> 11(tail) -->
    myList.displayBackwards();

    myList.deleteTail();
    myList.display(); // <-- 10(head) <--> 7 <--> 13(tail) -->
    myList.displayBackwards();

    myList.delete(7);
    myList.display(); // <-- 10(head) <--> 13(tail) -->
    myList.displayBackwards();

    myList.insertOrdered(23);
    myList.insertOrdered(67);
    myList.insertOrdered(3);
    myList.display(); // <-- 3(head) <--> 10 <--> 13 <--> 23 <--> 67(tail) -->
    myList.insertElementByIndex(5, 1);
    myList.display(); // <-- 3(head) <--> 5 <--> 10 <--> 13 <--> 23 <--> 67(tail) -->
    myList.displayBackwards();
    myList.reverse(); // <-- 67(head) <--> 23 <--> 13 <--> 10 <--> 5 <--> 3(tail) -->
    myList.display();
    
    myList.clearList();
    myList.display();
    myList.displayBackwards();
    myList.insertHead(20);
    myList.display();
    myList.displayBackwards();
  }
}