What is a Stack?

A stack is a data structure that stores a sequence of elements, but unlike an array or linked list, we can only access elements from one end, which is the top.

This is called LIFO (Last In, First Out). The last element we put in is the first one that gets out, like a stack of pancakes.

Structure

        TOP
         |
         v
        [4]   <-- most recently pushed, first to come out
         |
         v
        [3]
         |
         v
        [2]
         |
         v
        [1]   <-- first pushed, last to come out

Elements can only be pushed onto the top or popped off the top. We cannot access elements in the middle or bottom directly.

Core Operations

Operation	Description	Time
push(x)	Add an element to the top	O(1)
pop()	Remove and return the top element	O(1)
peek()	Return the top element without removing it	O(1)
isEmpty()	Check if the stack is empty	O(1)

When to use a Stack

• You need to reverse a sequence of elements
• You need to track history and undo operations
• You are parsing nested structures like brackets or HTML tags
• You need to implement recursive algorithms iteratively