javascript 链表(lineked lists)算法与说明

链表的介绍

链表存储有序的元素集合，但不同于数组，链表中的元素在内存中并不是连续放置的。每个元素由一个存储元素本身的节点和一个指向下一个元素的引用（也称为指针或链接）组成。下图讲解：

相对于传统的数组，链表的一个好处在于，添加或移除元素的时候不需要移动其他元素。然而，链表需要使用指针，因此实现链表的时候需要注意。数组的另一个细节是可以直接访问任何位置元素，而要想访问链表中间的一个元素，需要从起点（表头）开始送达列表直到找到所需要元素。

现实实例就是火车，火车有多节车厢相互衔接，通过接轨来链接火车。车厢就是链表的元素，而接轨就是指针。

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链表算法实例

1.链表增删改查。

let list = new LinkedList();
list.append(15);
list.append(10);
list.append(13);//15,10,13
list.indexOf(10);//1
list.removeAt(1)//15,13
list.insert(1,17);//15,17,13
list.remove(11);//17,13

ES6链表实现代码：

let LinkedList = (function () {

    class Node {//一个节点
        constructor(element){
            this.element = element;
            this.next = null;
        }
    }

    const length = new WeakMap();
    const head = new WeakMap();

    class LinkedList {
        constructor () {
            length.set(this, 0);
            head.set(this, null);
        }

        append(element) {//向列表尾部添加一个新的项
            let node = new Node(element),
                current;

            if (this.getHead() === null) { 
                head.set(this, node);
            } else {

                current = this.getHead();

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

                current.next = node;
            }

            let l = this.size();
            l++;
            length.set(this, l);
        }

        insert(position, element) {//向列表的特定位置插入一个新的项

            if (position >= 0 && position <= this.size()) {

                let node = new Node(element),
                    current = this.getHead(),
                    previous,
                    index = 0;

                if (position === 0) { 
                    node.next = current;
                    head.set(this, node);
                } else {
                    while (index++ < position) {
                        previous = current;
                        current = current.next;
                    }
                    node.next = current;
                    previous.next = node;
                }
                let l = this.size();
                l++;
                length.set(this, l);

                return true;

            } else {
                return false;
            }
        }

        removeAt(position) {//从列表的特定位置移除一项
            if (position > -1 && position < this.size()) {

                let current = this.getHead(),
                    previous,
                    index = 0;

                if (position === 0) {
                    head.set(this, current.next);
                } else {

                    while (index++ < position) {

                        previous = current;
                        current = current.next;
                    }

                    previous.next = current.next;
                }

                let l = this.size();
                l--;
                length.set(this, l);

                return current.element;

            } else {
                return null;
            }
        }

        remove(element) {//移除一项

            let index = this.indexOf(element);
            return this.removeAt(index);
        }

        indexOf(element) {//返回元素在列表中的索引，没有返回-1

            let current = this.getHead(),
                index = 0;

            while (current) {
                if (element === current.element) {
                    return index;
                }
                index++;
                current = current.next;
            }

            return -1;
        }

        isEmpty() {//链表不包含任何元素，返回true，如果长度大于0返回false
            return this.size() === 0;
        }

        size() {//返回链表包含的元素个数
            return length.get(this);
        }

        getHead() {//获取链头项
            return head.get(this);
        }

        toString() {//已string输出链表
            let current = this.getHead(),
                string = '';

            while (current) {
                string += current.element + (current.next ? ', ' : '');
                current = current.next;
            }
            return string;

        }

        print() {//string格式打印到控制台上。
            console.log(this.toString());
        }
    }

    return LinkedList;
})();

双向链表

双向链表和普通链表的区别在于。在链表中，一个节点只有链向下一个节点的链接，而在双向链表中，链接是双向的：一个链向下一个元素，另一个链向前一个元素。如下图：

双向链表提供了两种送代列表的方法：从头到尾，或者反过来。我们也可以访问一个特定节点的下一个或前一个元素。在单向链表中，如果送代列表时错过了要找的元素，就需要回到列表起点，重新开始送代。这是双向链表的一个优点。


事例：

let list = new DoublyLinkedList();
list.append(15);
list.append(16);//15,16
list.print();//15,16
list.printInverse();//16, 15
list.insert(0,13);//13,15,16
list.removeAt(0);//15,16

ES6代码实例：

let DoublyLinkedList = (function () {

    class Node {
        constructor(element) {
            this.element = element;
            this.next = null;
            this.prev = null; //NEW
        }
    }

    const length = new WeakMap();
    const head = new WeakMap();
    const tail = new WeakMap(); //NEW

    class DoublyLinkedList {

        constructor () {
            length.set(this, 0);
            head.set(this, null);
            tail.set(this, null);
        }

        append(element) {

            let node = new Node(element),
                current, _tail;

            if (this.getHead() === null) { //first node on list
                head.set(this, node);
                tail.set(this, node); //NEW
            } else {
                //attach to the tail node //NEW
                _tail = this.getTail();
                _tail.next = node;
                node.prev = _tail;
                tail.set(this, node);
            }

            //update size of list
            let l = this.size();
            l++;
            length.set(this, l);
        }

        insert(position, element) {

            //check for out-of-bounds values
            if (position >= 0 && position <= this.size()) {

                let node = new Node(element),
                    current = this.getHead(),
                    previous,
                    index = 0;

                if (position === 0) { //add on first position

                    if (!this.getHead()) {       //NEW
                        head.set(this, node);
                        tail.set(this, node);
                    } else {
                        node.next = current;
                        current.prev = node; //NEW {1}
                        head.set(this, node);
                    }

                } else if (position === this.size()) { //last item //NEW

                    current = tail;     // {2}
                    current.next = node;
                    node.prev = current;
                    tail.set(this, node);

                } else {
                    while (index++ < position) { //{3}
                        previous = current;
                        current = current.next;
                    }
                    node.next = current;
                    previous.next = node;

                    current.prev = node; //NEW
                    node.prev = previous; //NEW
                }

                //update size of list
                let l = this.size();
                l++;
                length.set(this, l);

                return true;

            } else {
                return false;
            }
        }

        removeAt(position) {

            //check for out-of-bounds values
            if (position > -1 && position < this.size()) {

                let _head = this.getHead(),
                    _tail = this.getTail(),
                    current = _head,
                    previous,
                    index = 0;

                //removing first item
                if (position === 0) {

                    _head = current.next; // {1}

                    //if there is only one item, then we update tail as well //NEW
                    if (this.size() === 1) { // {2}
                        _tail = null;
                    } else {
                        _head.prev = null; // {3}
                    }

                } else if (position === this.size() - 1) { //last item //NEW

                    current = _tail; // {4}
                    _tail = current.prev;
                    _tail.next = null;

                } else {

                    while (index++ < position) { // {5}

                        previous = current;
                        current = current.next;
                    }

                    //link previous with current's next - skip it to remove
                    previous.next = current.next; // {6}
                    current.next.prev = previous; //NEW
                }

                head.set(this,_head);
                tail.set(this,_tail);

                //update size of list
                let l = this.size();
                l--;
                length.set(this, l);

                return current.element;

            } else {
                return null;
            }
        }

        remove(element) {

            let index = this.indexOf(element);
            return this.removeAt(index);
        }

        indexOf(element) {

            let current = this.getHead(),
                index = -1;

            //check first item
            if (element == current.element) {
                return 0;
            }

            index++;

            //check in the middle of the list
            while (current.next) {

                if (element == current.element) {
                    return index;
                }

                current = current.next;
                index++;
            }

            //check last item
            if (element == current.element) {
                return index;
            }

            return -1;
        }

        isEmpty() {
            return this.size() === 0;
        }

        size() {
            return length.get(this);
        }

        toString() {

            let current = this.getHead(),
                s = current ? current.element : '';

            while (current && current.next) {
                current = current.next;
                s += ', ' + current.element;
            }

            return s;
        }

        inverseToString() {

            let current = this.getTail(),
                s = current ? current.element : '';

            while (current && current.prev) {
                current = current.prev;
                s += ', ' + current.element;
            }

            return s;
        }

        print() {
            console.log(this.toString());
        }

        printInverse() {
            console.log(this.inverseToString());
        }

        getHead() {
            return head.get(this);
        }

        getTail() {
            return tail.get(this);
        }
    }
    return DoublyLinkedList;
})();

循环链表

循环链表。就是最后一个元素指向不是null，而是指向第一个元素（head）。如下图。

实例用法：

let circularLinkedList = new CircularLinkedList();
circularLinkedList.append(15);
circularLinkedList.append(16);//15,16
circularLinkedList.insert(0,14);//14,15,16
circularLinkedList.removeAt(0);//15,16
circularLinkedList.indexOf(16)//1

ES6循环链表实例：

let CircularLinkedList = (function () {

    class Node {
        constructor(element) {
            this.element = element;
            this.next = null;
        }
    }

    const length = new WeakMap();
    const head = new WeakMap();

    class CircularLinkedList {

        constructor () {
            length.set(this, 0);
            head.set(this, null);
        }

        append(element) {

            let node = new Node(element),
                current;

            if (this.getHead() === null) { //first node on list
                head.set(this, node);
            } else {

                current = this.getHead();

                //loop the list until find last item
                while (current.next !== this.getHead()) { //last element will be head instead of NULL
                    current = current.next;
                }

                //get last item and assign next to added item to make the link
                current.next = node;
            }

            //set node.next to head - to have circular list
            node.next = this.getHead();

            //update size of list
            let l = this.size();
            l++;
            length.set(this, l);
        }

        insert(position, element) {

            //check for out-of-bounds values
            if (position >= 0 && position <= this.size()) {

                let node = new Node(element),
                    current = this.getHead(),
                    previous,
                    index = 0;

              if (position === 0) { //add on first position

                    node.next = current;

                    //update last element
                    while (current.next !== this.getHead()) { //last element will be head instead of NULL
                        current = current.next;
                    }

                    head.set(this, node);
                    current.next = this.getHead();

                } else {
                    while (index++ < position) {
                        previous = current;
                        current = current.next;
                    }
                    node.next = current;
                    previous.next = node;
                }

                //update size of list
                let l = this.size();
                l++;
                length.set(this, l);

                return true;

            } else {
                return false;
            }
        }

        removeAt(position) {

            //check for out-of-bounds values
            if (position > -1 && position < this.size()) {

                let current = this.getHead(),
                    previous,
                    index = 0;

                //removing first item
                if (position === 0) {

                    while (current.next !== this.getHead()) { //needs to update last element first
                        current = current.next;
                    }

                    head.set(this, this.getHead().next);
                    current.next = this.getHead();

                } else { //no need to update last element for circular list

                    while (index++ < position) {

                        previous = current;
                        current = current.next;
                    }

                    //link previous with current's next - skip it to remove
                    previous.next = current.next;
                }

                let l = this.size();
                l--;
                length.set(this, l);

                return current.element;

            } else {
                return null;
            }
        }

        remove(element) {

            let index = indexOf(element);
            return removeAt(index);
        }

        indexOf(element) {

            let current = this.getHead(),
                index = -1;

            //check first item
            if (element == current.element) {
                return 0;
            }

            index++;

            //check in the middle of the list
            while (current.next !== this.getHead()) {

                if (element == current.element) {
                    return index;
                }

                current = current.next;
                index++;
            }

            //check last item
            if (element == current.element) {
                return index;
            }

            return -1;
        }

        isEmpty() {
            return this.size() === 0;
        }

        size() {
            return length.get(this);
        }

        getHead() {
            return head.get(this);
        }

        toString() {

            let current = this.getHead(),
                s = current.element;

            while (current.next !== this.getHead()) {
                current = current.next;
                s += ', ' + current.element;
            }

            return s.toString();
        }

        print() {
            console.log(this.toString());
        }
    }
    return CircularLinkedList;
})();