What is a container that acts as both a FIFO and LIFO?

A container that can operate as both a FIFO (First In, First Out) and LIFO (Last In, First Out) is known as a doubly ended queue, or deque. This data structure allows for insertion and removal of elements from both ends, meaning you can add or remove items from either the front or the back.

When used in a FIFO manner, elements are added to one end (the back) and removed from the other end (the front), similar to how a standard queue functions. Conversely, when used as a LIFO structure, elements can be added and removed from the same end (the back), similar to how a stack operates.

This flexibility in element management is what makes the deque unique and valuable. It can adapt to various scenarios where either queue or stack behavior is required, providing an efficient and versatile option for developers. The other structures—stacks, queues, and trees—do not support this dual functionality.