What information is contained in the data structure stack?

A stack is a data structure that stores elements in LIFO (Last In, First Out) order, meaning the last added element is retrieved first. A stack contains a set of elements where two main operations can be performed:

• push — adding an element to the top of the stack;
• pop — removing and retrieving the top element.

Additionally, the peek (or top) operation is often implemented, which allows viewing the top element without removing it.

Example of using a stack in Swift:

var stack = [Int]()
stack.append(10)    // push
stack.append(20)
let top = stack.last  // peek, top = 20
let popped = stack.popLast()  // pop, popped = 20

Thus, a stack contains an ordered set of elements with access only to the most recently added element.