# Stacks

Stacks are the** linear data structure** which stores **data** in an **ordered** way.The **first element** inserted is at the **bottom** and **last elemen**t is at the **top**.It can be implemented using an **array or a linked list**.It is a **LIFO(Last In First Out)** data structure.This means that last element inserted will be first element to be removed incase of deletion operation(Fig 1,2).

**Examples of Stack**

- Stack of plates.The last plate added is the first one to be removed.
- A stack of files(Fig 1) .
- A girl wearing bangles.The last bangle on the arm is the first one to be removed.
- Luggage placed in truck.

### Array Representation of Stack

In **stack**,the **first element** is at** bottom**(0 position) and **last element** at **TOP** position.The topmost element of stack is represented by** TOP** variable.**MAX** variable is used to store the** maximum** no of** elements** that the** stack can** **hold**(Fig 3).

- if
**TOP =MAX-1**,then stack is**full**. - if
**TOP = NULL**,then stack is**empty**.

### Operations on Stack

Each function in a stack runs in O(1) time.

- Push - Refer Push Operation in Stacks
- Pop - Refer Pop Operation in Stacks
- Peep - return topmost value

### PEEP Operation

This operation first checks whether the** stack is empty** or has some elements(if TOP == NULL) and returns the **value** of **topmost element** **withou**t **deleting** it from the stack.In Fig 10, PEEP operation will **return 100** to user.

### Applications of Stack

- Converting a decimal number into binary.
- Towers of Hanoi
- Evaluation of expression and syntax parsing.

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