--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.

--hints--

Your LinkedList class should have a removeAt method.

assert(
  (function () {
    var test = new LinkedList();
    return typeof test.removeAt === 'function';
  })()
);

Your removeAt method should reduce the length of the linked list by one.

assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    test.add('dog');
    test.add('kitten');
    test.removeAt(1);
    return test.size() === 2;
  })()
);

Your removeAt method should remove the element at the specified index from the linked list.

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}}}'
    );
  })()
);

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.

assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    var removedItem = test.removeAt(0);
    return test.head() === null && test.size() === 0 && removedItem === 'cat';
  })()
);

Your removeAt method should return the element of the removed node.

assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    test.add('dog');
    test.add('kitten');
    return test.removeAt(1) === 'dog';
  })()
);

Your removeAt method should return null and the linked list should not change if the given index is less than 0.

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}}}'
    );
  })()
);

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.

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}}}'
    );
  })()
);

--seed--

--seed-contents--

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
}

--solutions--

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;
  };
}