std :: list c ++是顺序的，然后如何在序列中的任何位置进行插入和擦除操作的恒定时间

Question

in c++ reference i read "Lists are sequence containers that allow constant time insert and erase operations anywhere within the sequence, and iteration in both directions." my doubt is if it is sequential then how it can take constant time to delete and insert a node.Any way we have to traverse sequentially to reach that node . Deleting node depends on its position

Answer 1

O(1) referst to the complexity for inserting/erasing nodes provided you already have a handle (in the form of an iterator) to that node . Obtaining an iterator to the i th element of a list given an iterator to the first one is O(N).

This is quite often overlooked when judging the relative merits of std::list vs. say, std::vector . But note that both inserting and erasing elements return iterators that can be used for further insert/erase operations.

Answer 2

Insertion before a given node (lets call it pos ) or as last node or deleting a given node is a constant time operation.

std::list can be implemented as doubly linked list. Sequence containers should not be confused with contiguous memory requirement.

Answer 3

Because the list is not sorted. If you look closely, the actual function for putting elements in the list is list::insert(hint, element) ( http://www.cplusplus.com/reference/list/list/insert/ ). Ie for every insertion, the place where the element is added is already known, thus constant time.

Eg list::push_front(element) is short for insert(begin(), element) .

Answer 4

[That's mostly a comment, but it didn't fit in the margin]

Note that an std::list cannot be implemented as a single linked list to fulfill the complexity requirements:

• Even when you already have an handle to a node of a single linked list, you cannot insert before it (resp. erase it) in constant time (since you would need access to the previous node).

That's why std::forward_list only provides insert_after and erase_after .