js 实现数据结构 -- 链表

原文：

　　在 Javascript 中学习数据结构与算法。

概念： 

　　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(也称指针或链接)组成。下图展示了链表的结构: 

　　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，因此实现链表时需要额外注意。 数组的另一个细节是可以直接访问任何位置的任何元素，而要想访问链表中间的一个元素，需要从起点(表头)开始迭代列表直到找到所需的元素。

普通链表：

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

// 链表
class LinkedList {
    constructor() {
        this.head = null;
        this.length = 0; // length 同数组 length 与下标关系
    }

    // 追加元素
    append(element) {
        let node = new Node(element);
        let current = null;  // 指针？

        if (this.head === null) {
            this.head = node;
        } else {
            current = this.head;
            while (current.next) {
                current = current.next;
            }
            current.next = node;
        }
        this.length++;
    }

    // 任意位置插入元素
    insert (position, element) {
        if (position >= 0 && position <= this.length) {
            let node = new Node(element);
            let current = this.head;
            let previous = null;
            let index = 0;
            if (position === 0) {
                this.head = node;
            } else {
                while (index++ < position) {
                    previous = current;
                    current = current.next;
                }
                node.next = current;
                previous.next = node;
            }
            this.length++;
            return true
        } 
        return false
    }

    // 移除指定位置元素
    removeAt(position) {
        if (position > -1 && position < length) {
            let current = this.head;
            let previous = null;
            let index = 0;
            if (position === 0) {
                this.head = current.next;
            } else {
                while(index++ < position) {
                    previous = current;
                    current = current.next;
                }
                previous.next = current.next;
            }
            this.length--;
            return current.element;
        }
        return null
    }

    // 寻找元素下标
    findIndex(element) {
        let current = this.head;
        let index = -1;
        while (current) {
            if (element === current.element) {
                return index + 1;
            }
            index++;
            current = current.next;
        }

        return -1;
    }

    // 删除指定文档
    remove(element) {
        let index = this.findIndex(element);
        return removeAt(index);
    }

    isEmpty() {
        return !this.length;
    }

    size() {
        return this.length;
    }

    // 输出字符串
    toString() {
        let current = this.head;
        let string = '';
        while (current) {
            string += ` ${current.element}`;
            current = current.next;
        }
        return string;
    }
}

var ll = new LinkedList();
console.log(ll);
ll.append(2);
ll.append(6);
ll.append(24);
ll.append(152);

ll.insert(3, 18);
console.log(ll);
console.log(ll.findIndex(24));

双向链表：

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

// 双向链表
class DoubleLinkedList {
    constructor() {
        this.head = null;
        this.tail = null;
        this.length = 0;
    }

    // 任意位置插入元素
    insert(position, element) {
        if (position >= 0 && position <= ehis.length) {
            let node = new Node(element);
            let current = this.head;
            let previous = null;
            this.index = 0;
            // 首位
            if (position === 0) {
                if (!head) {
                    this.head = node;
                    this.tail = node;
                } else {
                    node.next = current;
                    this.head = node;
                    current.prev = node;
                }
            } else if (position === this.length) { // 末尾
                current = this.tail;
                current.next = node;
                node.prev = current;
                this.tail = node;
            } else {  // 中间
                while(index++ < position) {
                    previous = current;
                    current = current.next;
                }
                node.next = current;
                previous.next = node;
                current.prev = node;
                node.prev = previous;
            }
            this.length++;
            return true;
        }
        return false;
    }
    // 移除指定位置元素
    removeAt(position) {
        if (position > -1 && position < this.length) {
            let current = this.head;
            let previous = null;
            let index = 0;

            // 首位
            if (position === 0) {
                this.head = this.head.next
                this.head.prev = null
                if (this.length === 1) {
                    this.tail = null
                }
            } else if (position === this.length - 1) { // 末位
                this.tail = this.tail.prev
                this.tail.next = null
            } else { // 中位
                while (index++ < position) {
                     previous = current
                     current = current.next
                }
                previous.next = current.next
                current.next.prev = previous
            }
            this.length--;
            return current.element;
        } else {
            return null;
        }
    }

    // 其他方法
}

循环链表：

　　具体代码实现就不写了，下图是示意图