243 lines
5.8 KiB
Markdown
243 lines
5.8 KiB
Markdown
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---
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id: 587d8251367417b2b2512c65
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title: Remove Elements from a Linked List by Index
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challengeType: 1
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forumTopicId: 301711
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dashedName: remove-elements-from-a-linked-list-by-index
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---
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# --description--
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Before we move on to another data structure, let's get a couple of last bits of practice with linked lists.
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Let's write a `removeAt` method that removes the `element` at a given `index`. The method should be called `removeAt(index)`. To remove an `element` at a certain `index`, we'll need to keep a running count of each node as we move along the linked list.
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A common technique used to iterate through the elements of a linked list involves a <dfn>'runner'</dfn>, or sentinel, that 'points' at the nodes that your code is comparing. In our case, starting at the `head` of our list, we start with a `currentIndex` variable that starts at `0`. The `currentIndex` should increment by one for each node we pass.
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Just like our `remove(element)` method, which [we covered in a previous lesson](/learn/coding-interview-prep/data-structures/remove-elements-from-a-linked-list), we need to be careful not to orphan the rest of our list when we remove the node in our `removeAt(index)` method. We keep our nodes contiguous by making sure that the node that has reference to the removed node has a reference to the next node.
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# --instructions--
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Write a `removeAt(index)` method that removes and returns a node at a given `index`. The method should return `null` if the given `index` is either negative, or greater than or equal to the `length` of the linked list.
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**Note:** Remember to keep count of the `currentIndex`.
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# --hints--
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Your `LinkedList` class should have a `removeAt` method.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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return typeof test.removeAt === 'function';
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})()
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);
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```
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Your `removeAt` method should reduce the `length` of the linked list by one.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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test.add('dog');
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test.add('kitten');
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test.removeAt(1);
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return test.size() === 2;
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})()
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);
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```
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Your `removeAt` method should remove the element at the specified index from the linked list.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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test.add('dog');
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test.add('kitten');
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test.add('bird');
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test.removeAt(1);
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return (
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JSON.stringify(test.head()) ===
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'{"element":"cat","next":{"element":"kitten","next":{"element":"bird","next":null}}}'
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);
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})()
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);
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```
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When only one element is present in the linked list, your `removeAt` method should remove and return the element at specified index, and reduce the length of the linked list.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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var removedItem = test.removeAt(0);
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return test.head() === null && test.size() === 0 && removedItem === 'cat';
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})()
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);
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```
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Your `removeAt` method should return the element of the removed node.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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test.add('dog');
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test.add('kitten');
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return test.removeAt(1) === 'dog';
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})()
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);
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```
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Your `removeAt` method should return `null` and the linked list should not change if the given index is less than `0`.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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test.add('dog');
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test.add('kitten');
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var removedItem = test.removeAt(-1);
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return (
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removedItem === null &&
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JSON.stringify(test.head()) ===
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'{"element":"cat","next":{"element":"dog","next":{"element":"kitten","next":null}}}'
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);
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})()
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);
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```
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Your `removeAt` method should return `null` and the linked list should not change if the given index is greater than or equal to the `length` of the list.
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```js
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assert(
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(function () {
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var test = new LinkedList();
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test.add('cat');
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test.add('dog');
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test.add('kitten');
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var removedItem = test.removeAt(3);
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return (
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removedItem === null &&
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JSON.stringify(test.head()) ===
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'{"element":"cat","next":{"element":"dog","next":{"element":"kitten","next":null}}}'
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);
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})()
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);
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```
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# --seed--
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## --seed-contents--
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```js
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function LinkedList() {
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var length = 0;
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var head = null;
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var Node = function(element){
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this.element = element;
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this.next = null;
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};
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this.size = function(){
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return length;
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};
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this.head = function(){
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return head;
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};
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this.add = function(element){
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var node = new Node(element);
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if(head === null){
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head = node;
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} else {
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var currentNode = head;
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while(currentNode.next){
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currentNode = currentNode.next;
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}
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currentNode.next = node;
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}
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length++;
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};
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// Only change code below this line
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// Only change code above this line
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}
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```
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# --solutions--
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```js
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function LinkedList() {
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var length = 0;
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var head = null;
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var Node = function (element) {
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this.element = element;
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this.next = null;
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};
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this.size = function () {
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return length;
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};
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this.head = function () {
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return head;
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};
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this.add = function (element) {
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var node = new Node(element);
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if (head === null) {
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head = node;
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} else {
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var currentNode = head;
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while (currentNode.next) {
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currentNode = currentNode.next;
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}
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currentNode.next = node;
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}
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length++;
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};
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this.removeAt = function (index) {
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var currentNode = head;
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var previous = head;
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var count = 0;
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if (index >= length || index < 0) {
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return null;
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}
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if (index === 0) {
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var removed = head.element;
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head = currentNode.next;
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} else {
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while (count < index) {
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previous = currentNode;
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currentNode = currentNode.next;
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count++;
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}
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var removed = previous.next.element;
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previous.next = currentNode.next;
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}
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length--;
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return removed;
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};
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}
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```
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