container实现对集合的操作
Element是列表元素, 有指向前后元素的指针(next,prev)和一个指向当前列表的指针(list)
// Element is an element of a linked list.
type Element struct {
// Next and previous pointers in the doubly-linked list of elements.
// To simplify the implementation, internally a list l is implemented
// as a ring, such that &l.root is both the next element of the last
// list element (l.Back()) and the previous element of the first list
// element (l.Front()).
next, prev *Element
// The list to which this element belongs.
list *List
// The value stored with this element.
Value interface{}
}
List双向链表结构, root是链表的当前元素指针, len是链表长度
// List represents a doubly linked list.
// The zero value for List is an empty list ready to use.
type List struct {
root Element // sentinel list element, only &root, root.prev, and root.next are used
len int // current list length excluding (this) sentinel element
}