欣迪

Design your implementation of the linked list. You can choose to use a singly or doubly linked list.
A node in a singly linked list should have two attributes: val and nextval is the value of the current node, and next is a pointer/reference to the next node.
If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement the MyLinkedList class:

  • MyLinkedList() Initializes the MyLinkedList object.
  • int get(int index) Get the value of the indexth node in the linked list. If the index is invalid, return -1.
  • void addAtHead(int 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.
  • void addAtTail(int val) Append a node of value val as the last element of the linked list.
  • void addAtIndex(int index, int val) Add a node of value val before the indexth node in the linked list. If index equals the length of the linked list, the node will be appended to the end of the linked list. If index is greater than the length, the node will not be inserted.
  • void deleteAtIndex(int index) Delete the indexth node in the linked list, if the index is valid.

Example 1:

Input
["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"]
[[], [1], [3], [1, 2], [1], [1], [1]]
Output

[null, null, null, null, 2, null, 3]

Explanation MyLinkedList myLinkedList = new MyLinkedList(); myLinkedList.addAtHead(1); myLinkedList.addAtTail(3); myLinkedList.addAtIndex(1, 2); // linked list becomes 1->2->3 myLinkedList.get(1); // return 2 myLinkedList.deleteAtIndex(1); // now the linked list is 1->3 myLinkedList.get(1); // return 3

Constraints:

  • 0 <= index, val <= 1000
  • Please do not use the built-in LinkedList library.
  • At most 2000 calls will be made to getaddAtHeadaddAtTailaddAtIndex and deleteAtIndex.

const Node = function (val) {
    this.val = val
    this.next = null
}

var MyLinkedList = function() {
    this.head = null
    this.tail = null
    this.size = 0
};

/** 
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.get = function(index) {
    if (this.size === 0 || index < 0 || index > this.size - 1) {
        return -1
    }
    let target = this.head
    let i = 0
    while(i < index) {
        target = target.next
        i++
    }
    return target.val
};

/** 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function(val) {
    const node = new Node(val)
    if (!this.size) {
        this.head = node
        this.tail = node
    } else {
        node.next = this.head
        this.head = node
    }
    
    this.size++
};

/** 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function(val) {
    const node = new Node(val)
    if (!this.size) {
        this.head = node
        this.tail = node
    } else {
        this.tail.next = node
        this.tail = node
    }

    this.size++
};

/** 
 * @param {number} index 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function(index, val) {
    if (index < 0 || index > this.size) {
        return
    }
    if (index === this.size) {
        this.addAtTail(val)
        return
    }
    if (index === 0) {
        this.addAtHead(val)
        return
    }
    let target = this.head
    let i = 0
    while(i < index - 1) {
        i++
        target = target.next
    }
    const node = new Node(val)
    node.next = target.next ? target.next : null
    target.next = node
    this.size++
};

/** 
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function(index) {
    if (index < 0 || index > this.size - 1) {
        return
    }
    
    if(index === 0) {
        this.head = this.head.next
        this.size--
        return
    }
    let i = 0
    let target = this.head
    while(i < index - 1) {
        i++
        target = target.next
    }
    target.next = target.next.next
    if (!target.next) {
        this.tail = target
    }
    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)
 */

第一次觸碰時,花了不少時間去搞懂 LinkedList。 弄懂之後,發現概念上其實沒那麼複雜。

首先必須了解這個資料結構的基礎原理,如下圖:

在 js 中,必須要有一個 Node 的模型,和一個用來整合的 LinkedList 模型。

// 基本的 Node 模型
const Node = function (val) {
    this.val = val
    this.next = null
}

而 LinkedList 則是用來整合,記錄這些節點,通常我們只需要知道起始節點(head)就可以做計算。

這題為了計算方便,增加了 size (節點數量) 和 tail (結束節點)

var MyLinkedList = function() {
    this.head = null
    this.tail = null
    this.size = 0
};

增加 tail 有一個明顯的好處就是在處理增加結尾節點(addAtTail)時,可以不用從 head 開始往下找。

 MyLinkedList.prototype.addAtTail = function(val) {
    const node = new Node(val)
    if (!this.size) {
        this.head = node
        this.tail = node
    } else {
        this.tail.next = node
        this.tail = node
    }

    this.size++
};

選取節點時,通常使用一個 loop 去找到我們要選取的目標

MyLinkedList.prototype.get = function(index) {
    if (this.size === 0 || index < 0 || index > this.size - 1) {
        return -1
    }
    let target = this.head
    let i = 0
    while(i < index) {
        target = target.next
        i++
    }
    return target.val
};

插入和刪除時,就會發現在 LinkedList 增加 size 的好處,可以直接避免 input value 的錯誤。

MyLinkedList.prototype.addAtIndex = function(index, val) {
    if (index < 0 || index > this.size) {
        return
    }
    if (index === this.size) {
        this.addAtTail(val)
        return
    }
    if (index === 0) {
        this.addAtHead(val)
        return
    }
    let target = this.head
    let i = 0
    while(i < index - 1) {
        i++
        target = target.next
    }
    const node = new Node(val)
    node.next = target.next ? target.next : null
    target.next = node
    this.size++
};

/** 
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function(index) {
    if (index < 0 || index > this.size - 1) {
        return
    }
    
    if(index === 0) {
        this.head = this.head.next
        this.size--
        return
    }
    let i = 0
    let target = this.head
    while(i < index - 1) {
        i++
        target = target.next
    }
    target.next = target.next.next
    if (!target.next) {
        this.tail = target
    }
    this.size--
};

到這裡,基礎的 LinkedList 觀念已經差不多有個雛形了。

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作者介紹 - 欣迪

欣迪

從設計到寫程式,發現自己有追求前端技巧的自虐傾向。不斷的踩坑,再從坑裡爬出來,慢慢對攀岩有點心得。 目前在多間公司擔任網站設計顧問。 同時也是網站架設公司負責人。

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