/**
 * 双向链表
 */
 function DoublyLinkedList() {
        var Node = function (element) {
            this.element = element;
            this.next = null;
            this.prev = null;
        };
        var length = 0;
        var head = null;
        var tail = null;
        this.insert = function (position, element) {
            if (position >= 0 && position <= length) {
                var node = new Node(element);
                var current = head;
                var previous;
                var index = 0;
                if (position === 0) {
                    if (!head) {
                        head = node;
                        tail = node;
                    } else {
                        node.next = current;
                        current.prev = node;
                        head = node;
                    }
                } else if (position === length) {
                    current = tail;
                    current.next = node;
                    node.prev = current;
                    tail = node;
                } else {
                    while (index++ < position) {
                        previous = current;
                        current = current.next;
                    }
                    node.next = current;
                    previous.next = node;

                    current.prev = node;
                    node.prev = previous;
                }
                length++;
                return true;
            } else {
                return false;
            }
        };

        this.removeAt = function (position) {
            if (position > -1 && position < length) {
                var current = head,
                        previous,
                        index = 0;
                if (position === 0) {
                    head = current.next;
                    if (length === 1) {
                        tail = null;
                    } else {
                        head.prev = null;
                    }
                } else if (position === length - 1) {
                    current = tail;
                    tail = current.prev;
                    tail.next = null;
                } else {
                    while (index++ < position) {
                        previous = current;
                        current = current.next;
                    }
                    previous.next = current.next;
                    current.next.prev = previous;
                }
                length--;
                return current.element;
            } else {
                return null;
            }
        };

    }