159 lines
6.2 KiB
Markdown
159 lines
6.2 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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---
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## Description
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<section id='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 <code>removeAt</code> method that removes the <code>element</code> at a given <code>index</code>. The method should be called <code>removeAt(index)</code>. To remove an <code>element</code> at a certain <code>index</code>, 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 <code>head</code> of our list, we start with a <code>currentIndex</code> variable that starts at <code>0</code>. The <code>currentIndex</code> should increment by one for each node we pass.
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Just like our <code>remove(element)</code> method, which <a href="/learn/coding-interview-prep/data-structures/remove-elements-from-a-linked-list" target="_blank">we covered in a previous lesson</a>, we need to be careful not to orphan the rest of our list when we remove the node in our <code>removeAt(index)</code> 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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</section>
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## Instructions
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<section id='instructions'>
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Write a <code>removeAt(index)</code> method that removes and returns a node at a given <code>index</code>. The method should return <code>null</code> if the given <code>index</code> is either negative, or greater than or equal to the <code>length</code> of the linked list.
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<strong>Note:</strong> Remember to keep count of the <code>currentIndex</code>.
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: Your <code>LinkedList</code> class should have a <code>removeAt</code> method.
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testString: assert((function(){var test = new LinkedList(); return (typeof test.removeAt === 'function')}()));
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- text: Your <code>removeAt</code> method should reduce the <code>length</code> of the linked list by one.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); test.removeAt(1); return test.size() === 2}()));
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- text: Your <code>removeAt</code> method should remove the element at the specified index from the linked list.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); test.add('bird');test.removeAt(1); return JSON.stringify(test.head()) === '{"element":"cat","next":{"element":"kitten","next":{"element":"bird","next":null}}}'}()));
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- text: When only one element is present in the linked list, your <code>removeAt</code> method should remove and return the element at specified index, and reduce the length of the linked list.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); var removedItem = test.removeAt(0); return test.head() === null && test.size() === 0 && removedItem === 'cat';}()));
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- text: Your <code>removeAt</code> method should return the element of the removed node.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); return test.removeAt(1) === 'dog'}()));
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- text: Your <code>removeAt</code> method should return <code>null</code> and the linked list should not change if the given index is less than <code>0</code>.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); var removedItem = test.removeAt(-1); return removedItem === null && JSON.stringify(test.head()) === '{"element":"cat","next":{"element":"dog","next":{"element":"kitten","next":null}}}'}()));
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- text: Your <code>removeAt</code> method should return <code>null</code> and the linked list should not change if the given index is greater than or equal to the <code>length</code> of the list.
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testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); var removedItem = test.removeAt(3); return removedItem === null && JSON.stringify(test.head()) === '{"element":"cat","next":{"element":"dog","next":{"element":"kitten","next":null}}}'}()));
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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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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</div>
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</section>
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## Solution
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<section id='solution'>
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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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</section>
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