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设计链表

题目​

设计链表的实现。你可以选择使用单链表或双链表。单链表中的节点应该具有两个属性：val  和  next。

val  是当前节点的值，next  是指向下一个节点的指针/引用。如果要使用双向链表，则还需要一个属性  prev  以指示链表中的上一个节点。假设链表中的所有节点都是 0-index 的。

在链表类中实现这些功能：

  • get(index)：获取链表中第  index  个节点的值。如果索引无效，则返回-1。
  • addAtHead(val)：在链表的第一个元素之前添加一个值为  val  的节点。插入后，新节点将成为链表的第一个节点。
  • addAtTail(val)：将值为  val 的节点追加到链表的最后一个元素。
  • addAtIndex(index,val)：在链表中的第  index  个节点之前添加值为  val  的节点。如果  index  等于链表的长度，则该节点将附加到链表的末尾。如果 index 大于链表长度，则不会插入节点。如果 index 小于 0，则在头部插入节点。
  • deleteAtIndex(index)：如果索引  index 有效，则删除链表中的第  index 个节点。

示例：

const linkedList = new MyLinkedList();
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1, 2); //链表变为1-> 2-> 3
linkedList.get(1); //返回2
linkedList.deleteAtIndex(1); //现在链表是1-> 3
linkedList.get(1); //返回3

代码实现​

var node = function (x) {
  this.val = x;
  this.next = null;
};

/**
 * Initialize your data structure here.
 */
var MyLinkedList = function () {
  this.head = null;
  this.size = 0;
};

/**
 * Get the value of the index-th node in the linked list. If the index is invalid, return -1.
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.get = function (index) {
  if (index < 0 || index > this.size - 1) return -1;
  let cur = this.head;
  for (let i = 0; i < index; i++) {
    cur = cur.next;
  }
  return cur.val;
};

/**
 * Add a node of value val before the first element of the linked list.
 * After the insertion, the new node will be the first node of the linked list.
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function (val) {
  const newHead = new node(val);
  newHead.next = this.head;
  this.head = newHead;
  this.size++;
};

/**
 * Append a node of value val to the last element of the linked list.
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function (val) {
  const newNode = new node(val);
  if (this.head === null) {
    this.head = newNode;
  } else {
    let cur = this.head;
    while (cur.next !== null) {
      cur = cur.next;
    }
    cur.next = newNode;
  }
  this.size++;
};

/**
 * Add a node of value val before the index-th node in the linked list.
 * If index equals to the length of linked list, the node will be appended to the end of linked list.
 * If index is greater than the length, the node will not be inserted.
 * @param {number} index
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function (index, val) {
  if (index < 0 || index > this.size) {
    return;
  }
  if (index === 0) {
    this.addAtHead(val);
    return;
  }
  if (index === this.size) {
    this.addAtTail(val);
    return;
  }

  let cur = this.head;
  for (let i = 0; i < index - 1; i++) {
    cur = cur.next;
  }
  const newNode = new node(val);
  newNode.next = cur.next;
  cur.next = newNode;
  this.size++;
};

/**
 * Delete the index-th node in the linked list, if the index is valid.
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function (index) {
  if (index < 0 || index >= this.size) {
    return;
  }
  if (index === 0) {
    this.head = this.head.next;
    return;
  }

  let cur = this.head;
  for (let i = 0; i < index - 1; i++) {
    cur = cur.next;
  }
  cur.next = cur.next.next;

  this.size--;
};

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * var obj = new MyLinkedList()
 * var param_1 = obj.get(index)
 * obj.addAtHead(val)
 * obj.addAtTail(val)
 * obj.addAtIndex(index,val)
 * obj.deleteAtIndex(index)
 */