Stacks and Queues

A stack is last in, first out: push adds to the end and pop takes from the end, like a pile of plates. A queue is first in, first out: add with push and take from the front with shift. shift has to move every other item, so for big queues keep an index of the front instead. Stacks power undo and bracket matching; queues process work in arrival order.

How it reads

• pop returns the most recent item: the last action is undone first
• shift returns the oldest item: first come, first served
• Both change the original array

Cloud tip: Matching things in reverse order, like brackets or HTML tags, is a job for a stack.