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jaebradleyjaebradley
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update doubly linked list and add code review markdown
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‎codereview/DoublyLinkedList.md‎

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## Purpose
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I've never implemented a `DoublyLinkedList` and thought it would be a good data structure exercise. I tried using outside
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resources as little as possible. I also tried to keep things as simple as possible
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## Discussion
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The `DoublyLinkedList` is made up of a `Node` `head`, a `Node` `tail`, and an `int` `length`. I did not genericize the
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API to keep things simple - the API only deals with `int` values.
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The API is
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* `getHead` - gets the `Node` at the `head` of the linked list in `O(1)`
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* `getTail` - gets the `Node` at the `tail` of the linked list in `O(1)`
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* `getLength` - gets the `length` of the linked list in `O(1)`
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* `isEmpty` - convenience method; returns `true` if `length` is `0` and `false` otherwise in `O(1)`
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* `insertAtHead` - adds the input value at the `head` of the linked list in `O(1)`
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* `insertAtTail` - adds the input value at the `tail` of the linked list in `O(1)`
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* `insertAt` - adds the input value at some index in the linked list in `O(n)`
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* `removeFirstOccurrence` - removes the first occurrence of the input value (if it exists in list) in `O(n)`
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* `removeAt` - removes the node at the specified index in the linked list in `O(n)`
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* `removeHead` - removes the node at the `head` of the linked list in `O(1)`
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* `removeTail` - removes the node at the `tail` of the linked list in `O(1)`
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Things I can improve:
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* When inserting at / removing at a particular index, I could speed up execution time by picking to start
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at the beginning or end of the list based on which the index is closest to
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* I tried adding a bunch of test cases, but I could have easily missed a case
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* Is my API sane / reasonable / did I miss implementing any obvious methods?
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## Implementation
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<!-- language:lang-java --!>
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public class DoublyLinkedList {
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public static class Node {
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private Node previous;
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private Node next;
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private int value;
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public Node(int value) {
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this.value = value;
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}
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public Node getPrevious() {
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return previous;
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}
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public Node getNext() {
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return next;
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}
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public int getValue() {
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return value;
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}
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}
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private Node head;
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private Node tail;
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private int length;
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public DoublyLinkedList() {
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}
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public Node getHead() {
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return head;
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}
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public Node getTail() {
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return tail;
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}
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public int getLength() {
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return length;
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}
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public boolean isEmpty() {
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return length == 0;
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}
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public void insertAtHead(int value) {
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insertNode(null, new Node(value), head);
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}
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public void insertAtTail(int value) {
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insertNode(tail, new Node(value), null);
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}
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public void insertAt(int value, int index) {
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if (index > length || index < 0) {
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throw new IndexOutOfBoundsException("index of " + index + " is out of bounds");
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}
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Node node = new Node(value);
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Node previousNode = null;
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Node currentNodeAtIndex = head;
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for (int i = 0; i < index; i++) {
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previousNode = currentNodeAtIndex;
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currentNodeAtIndex = currentNodeAtIndex.next;
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}
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insertNode(previousNode, node, currentNodeAtIndex);
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}
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public void removeFirstOccurrence(int value) {
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Node currentNode = head;
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while (currentNode != null) {
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if (currentNode.value == value) {
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removeNode(currentNode);
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return;
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}
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currentNode = currentNode.next;
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}
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}
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public void removeAt(int index) {
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if (index >= length || index < 0) {
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throw new IndexOutOfBoundsException("index of " + index + " is out of bounds");
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}
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Node currentNodeAtIndex = head;
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for (int i = 0; i < index; i++) {
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currentNodeAtIndex = currentNodeAtIndex.next;
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}
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removeNode(currentNodeAtIndex);
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}
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public void removeHead() {
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removeNode(head);
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}
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public void removeTail() {
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removeNode(tail);
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}
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private void removeNode(Node node) {
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if (node.previous == null) {
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head = node.next;
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}
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if (node.next == null) {
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tail = node.previous;
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}
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if (node.previous != null) {
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node.previous.next = node.next;
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}
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if (node.next != null) {
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node.next.previous = node.previous;
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}
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length--;
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}
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private void insertNode(Node nodeBefore, Node node, Node nodeAfter) {
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node.next = nodeAfter;
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node.previous = nodeBefore;
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if (nodeBefore == null) {
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head = node;
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}
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if (nodeAfter == null) {
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tail = node;
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}
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if (nodeBefore != null) {
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nodeBefore.next = node;
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}
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if (nodeAfter != null) {
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nodeAfter.previous = node;
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}
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length++;
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}
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}

‎src/main/java/problems/impl/DoublyLinkedList.java‎

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private Node head;
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private Node tail;
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private int length = 0;
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private int length;
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public DoublyLinkedList() {
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}
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removeNode(currentNodeAtIndex);
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}
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public void removeHead() {
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removeNode(head);
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}
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public void removeTail() {
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removeNode(tail);
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}
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private void removeNode(Node node) {
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if (node.previous == null) {
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head = node.next;
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tail = node.previous;
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}
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if (node.previous != null && node.next != null) {
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if (node.previous != null) {
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node.previous.next = node.next;
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}
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if (node.next != null) {
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node.next.previous = node.previous;
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}
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length--;
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}
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private void insertNode(Node nodeBefore, Node node, Node nodeAfter) {
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node.next = nodeAfter;
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node.previous = nodeBefore;
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if (nodeBefore == null) {
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head = node;
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}
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tail = node;
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}
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node.next = nodeAfter;
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node.previous = nodeBefore;
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if (nodeBefore != null) {
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nodeBefore.next = node;
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}
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if (nodeAfter != null) {
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nodeAfter.previous = node;
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}
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length++;
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}

‎src/test/java/problems/impl/DoublyLinkedListTest.java‎

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assertNull(list.getHead());
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assertEquals(list.getHead(), list.getTail());
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}
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@Test
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public void removeHeadForSingleElementList() {
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DoublyLinkedList list = new DoublyLinkedList();
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list.insertAtHead(1);
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list.removeHead();
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assertEquals(0, list.getLength());
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assertNull(list.getHead());
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assertEquals(list.getHead(), list.getTail());
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}
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@Test
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public void removeHeadForTwoElementList() {
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DoublyLinkedList list = new DoublyLinkedList();
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list.insertAtHead(2);
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list.insertAtHead(1);
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list.removeHead();
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assertEquals(1, list.getLength());
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assertEquals(2, list.getHead().getValue());
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assertEquals(list.getHead(), list.getTail());
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}
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@Test
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public void removeTailForSingleElementList() {
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DoublyLinkedList list = new DoublyLinkedList();
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list.insertAtHead(1);
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list.removeTail();
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assertEquals(0, list.getLength());
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assertNull(list.getHead());
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assertEquals(list.getHead(), list.getTail());
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}
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@Test
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public void removeTailForTwoElementList() {
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DoublyLinkedList list = new DoublyLinkedList();
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list.insertAtHead(2);
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list.insertAtHead(1);
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list.removeTail();
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assertEquals(1, list.getLength());
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assertEquals(1, list.getHead().getValue());
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assertEquals(list.getHead(), list.getTail());
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}
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}

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