// https://www.freecodecamp.org/learn/coding-interview-prep/data-structures/search-within-a-linked-list

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.remove = function (element) {
    var currentNode = head;
    var previousNode;
    if (currentNode.element === element) {
      head = currentNode.next;
    } else {
      while (currentNode.element !== element) {
        previousNode = currentNode;
        currentNode = currentNode.next;
      }

      previousNode.next = currentNode.next;
    }

    length--;
  };

  // Only change code below this line
  this.isEmpty = () => {
    return head === null;
  };

  this.indexOf = (element) => {
    let i = 0;
    let node = head;
    while (node !== null && node.element !== element) {
      i++;
      node = node.next;
    }
    if (node === null) return -1;
    return i;
  };

  this.elementAt = (index) => {
    if (index < 0 || index >= length || head === null) {
      return undefined;
    }
    let i = 0;
    let node = head;
    while (i < index && node !== null) {
      i++;
      node = node.next;
    }
    if (i != index) {
      return undefined;
    }
    return node.element;
  };
  // Only change code above this line
}