freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/data-structures/remove-elements-from-a-linked-list-by-index.md

id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d8251367417b2b2512c65	Remove Elements from a Linked List by Index	1	301711

Description

Before we move on to another data structure, let's get a couple of last bits of practice with linked lists. 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. A common technique used to iterate through the elements of a linked list involves a 'runner', 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. Just like our remove(element) method, which we covered in a previous lesson, 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.

Instructions

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. Note: Remember to keep count of the currentIndex.

Tests

tests:
  - text: Your <code>LinkedList</code> class should have a <code>removeAt</code> method.
    testString: assert((function(){var test = new LinkedList(); return (typeof test.removeAt === 'function')}()));
  - text: Your <code>removeAt</code> method should reduce the <code>length</code> of the linked list by one.
    testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten'); test.removeAt(1); return test.size() === 2}()));
  - text: Your <code>removeAt</code> method should remove the element at the specified index from the linked list.
    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}}}'}())); 
  - 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.
    testString: assert((function(){var test = new LinkedList(); test.add('cat'); var removedItem = test.removeAt(0); return test.head() === null && test.size() === 0 && removedItem === 'cat';}()));        
  - text: Your <code>removeAt</code> method should return the element of the removed node.
    testString: assert((function(){var test = new LinkedList(); test.add('cat'); test.add('dog'); test.add('kitten');  return test.removeAt(1) === 'dog'}()));
  - 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>.
    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}}}'}()));
  - 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.
    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}}}'}()));

Challenge Seed

function LinkedList() {
  var length = 0;
  var head = null;

  var Node = function(element){
    this.element = element;
    this.next = null;
  };

  this.size = function(){
    return length;
  };

  this.head = function(){
    return head;
  };

  this.add = function(element){
    var node = new Node(element);
    if(head === null){
      head = node;
    } else {
      var currentNode = head;

      while(currentNode.next){
        currentNode  = currentNode.next;
      }

      currentNode.next = node;
    }

    length++;
  };

  // Only change code below this line

  // Only change code above this line
}

Solution

function LinkedList() {
  var length = 0;
  var head = null;

  var Node = function (element) {
    this.element = element;
    this.next = null;
  };

  this.size = function () {
    return length;
  };

  this.head = function () {
    return head;
  };

  this.add = function (element) {
    var node = new Node(element);
    if (head === null) {
      head = node;
    } else {
      var currentNode = head;

      while (currentNode.next) {
        currentNode = currentNode.next;
      }

      currentNode.next = node;
    }

    length++;
  };

  this.removeAt = function (index) {
    var currentNode = head;
    var previous = head;
    var count = 0;
    if (index >= length || index < 0) {
      return null;
    }
    if (index === 0) {
      var removed = head.element;
      head = currentNode.next;
    } else {
      while (count < index) {
        previous = currentNode;
        currentNode = currentNode.next;
        count++;
      }
      var removed = previous.next.element;
      previous.next = currentNode.next;
    }
    length--;
    return removed;
  };
}