A stack is a useful data structure in programming. It is just like a pile of books kept on top of each other.
Think about the things you can do with such a pile of plates
- Put a new plate on top
- Remove the top plate
If you want the plate at the bottom, you must first remove all the plates on top. Such an arrangement is called Last In First Out — the last item that is the first item to go out.
Basic Operations of Stack
A stack is an object (an abstract data type — ADT) that allows the following operations:
- Push: Add an element to the top of a stack
- Pop: Remove an element from the top of a stack
- IsEmpty: Check if the stack is empty
- GetStack: Returns the entire stack
- Peek: Get the value of the top element without removing it
Stack Implementations in Python
Stack Time Complexity
For the array-based implementation of a stack, the push and pop operations take constant time, i.e.
Applications of Stack Data Structure
Although stack is a simple data structure to implement, it is very powerful. The most common uses of a stack are:
- To reverse a word — Put all the letters in a stack and pop them out. Because of the LIFO order of stack, you will get the letters in reverse order.
- In compilers — Compilers use the stack to calculate the value of expressions like
2 + 4 / 5 * (7 - 9)by converting the expression to prefix or postfix form.
- In browsers — The back button in a browser saves all the URLs you have visited previously in a stack. Each time you visit a new page, it is added on top of the stack. When you press the back button, the current URL is removed from the stack, and the previous URL is accessed.