Javascript数据结构与算法（二）循环链表与有序链表

摘要：循环链表可以像单向链表引用，也可以像双向链表有双向引用。以下就以如何创建栈数据结构为例。

循环链表可以像单向链表引用，也可以像双向链表有双向引用。性能上也跟双向链表差不多，如果position大于length/2，那就可以从尾部开始迭代，可以减少迭代的元素。唯一的区别在于最后一个元素指向下一个元素的指针(tail.next)不是undefine，而是第一个元素（head）。接下来来看一下循环链表的代码

class CircularLinkedList extends LinkedList {
  constructor(equalsFn = defaultEquals) {
    super(equalsFn);
  }

push(element) {
    const node = new Node(element);
    let current;
    if (this.head == null) {
      this.head = node;
    } else {
      current = this.getElementAt(this.size() - 1);
      current.next = node;
    }
    // set node.next to head - to have circular list
    node.next = this.head;
    this.count++;
  }

insert(element, index) {
    if (index >= 0 && index <= this.count) {
      const node = new Node(element);
      let current = this.head;
      if (index === 0) {
        if (this.head == null) {
          // if no node  in list
          this.head = node;
          node.next = this.head;
        } else {
          node.next = current;
          current = this.getElementAt(this.size());
          // update last element
          this.head = node;
          current.next = this.head;
        }
      } else {
        const previous = this.getElementAt(index - 1);
        node.next = previous.next;
        previous.next = node;
      }
      this.count++;
      return true;
    }
    return false;
  }

removeAt(index) {
    if (index >= 0 && index < this.count) {
      let current = this.head;
      if (index === 0) {
        if (this.size() === 1) {
          this.head = undefined;
        } else {
          const removed = this.head;
          current = this.getElementAt(this.size() - 1);
          this.head = this.head.next;
          current.next = this.head;
          current = removed;
        }
      } else {
        // no need to update last element for circular list
        const previous = this.getElementAt(index - 1);
        current = previous.next;
        previous.next = current.next;
      }
      this.count--;
      return current.element;
    }
    return undefined;
  }
}

有序链表是指保持元素有序的；链表结构，除了使用排序算法之外，我们还可以将元素插入到正确的位置来保证链表的有序性。

const Compare = {
  LESS_THAN: -1,
  BIGGER_THAN: 1,
  EQUALS: 0
};
function defaultCompare(a, b) {
  if (a === b) {
    return Compare.EQUALS;
  }
  return a < b ? Compare.LESS_THAN : Compare.BIGGER_THAN;
};
function defaultEquals(a, b) {
  return a === b;
};
class SortedLinkedList extends LinkedList {
  constructor(equalsFn = defaultEquals, compareFn = defaultCompare) {
    super(equalsFn);
    this.equalsFn = equalsFn;
    this.compareFn = compareFn;
};

getIndexNextSortedElement(element) {
    let current = this.head;
    let i = 0;
    for (; i < this.size() && current; i++) {
      const comp = this.compareFn(element, current.element);
      if (comp === Compare.LESS_THAN) {
        return i;
      }
      current = current.next;
    }
    return i;
  }
}

push(element) {
    if (this.isEmpty()) {
      super.push(element);
    } else {
      const index = this.getIndexNextSortedElement(element);
      super.insert(element, index);
    }
  }

insert(element, index = 0) {
    if (this.isEmpty()) {
      return super.insert(element, index === 0 ? index : 0);
    }
    const pos = this.getIndexNextSortedElement(element);
    return super.insert(element, pos);
  }

以上就是常见的四种链表结构，当然我们也可以使用链表实现其他数据结构，例如栈，队列，双向队列。以下就以如何创建栈数据结构为例。

class StackLinkedList {
  constructor() {
    this.items = new DoublyLinkedList();
  }

  push(element) {
    this.items.push(element);
  }

  pop() {
    if (this.isEmpty()) {
      return undefined;
    }
    const result = this.items.removeAt(this.size() - 1);
    return result;
  }

  peek() {
    if (this.isEmpty()) {
      return undefined;
    }
    return this.items.getElementAt(this.size() - 1).element;
  }

  isEmpty() {
    return this.items.isEmpty();
  }

  size() {
    return this.items.size();
  }

  clear() {
    this.items.clear();
  }

  toString() {
    return this.items.toString();
  }
}

链表相对数组最大的优点就是无需移动链表中的元素，就能轻松的添加和移除元素，所以需要实现添加和移除元素操作时，最好的选择是链表而非数组。