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),
current = head,
previous,
index = 0;
if (position == 0) {
if (!head) {
head = node;
tail = node
} else {
node.next = current;
current.previous = 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
}
}
}