# What are Stacks in Data Structure?

A stack is a data structure that follows the Last In First Out (LIFO) principle. This means that the last item added to the stack will be the first one to be removed. It is often represented as a vertical list, with the most recently added item at the top and the oldest item at the bottom.

**A stack has two main operations: push and pop**. Push adds an item to the top of the stack, and pop removes the top item from the stack. Additionally, there is usually a peek operation which returns the top item without removing it from the stack.

It's important to note that there are other data structures that are similar to the stack, such as the queue and the deque (double-ended queue). A queue follows the First In First Out (FIFO) principle, and a deque allows items to be added and removed from both ends.

## Types of Stacks

There are several types of stacks, including:

**Simple Stack:**A basic stack data structure that follows the LIFO principle and has operations such as push, pop, and peek.**Array-based Stack:**A stack implementation using an array to store the items. The array's size can be fixed or dynamic, and it allows for fast random access to elements but has a fixed size limit.**Linked List-based Stack:**A stack implementation using a linked list to store the items. The linked list has no size limit, but operations on it tend to be slower than on an array-based stack.**Dynamic Stack:**A stack that can grow or shrink dynamically based on the number of items stored in it. This can be implemented using an array or a linked list.**Infix to Postfix/Prefix conversion:**Stacks are used in the infix to postfix/prefix conversion process. It's used in the evaluation of expressions in reverse polish notation.**Recursive Stack:**A stack that is used during the recursion process. Each recursive call adds a new frame to the stack, and each return pops a frame from the stack.**Memory Stack:**A stack that is used by the operating system to keep track of the memory allocated to different processes. It's also used in memory management and memory allocation.**Expression evaluation Stack:**It's used to evaluate expressions and solve mathematical problems such as postfix and prefix expressions.

It's worth noting that some types of stacks can be implemented using other data structures, such as Queue and Deque, thus leading to variations of the types of stacks.

## Example of Stacks

Here are examples of how to implement a stack in a few different programming languages:

```
#include <iostream>
#include <stack>
int main() {
std::stack<int> s;
// push elements onto the stack
s.push(1);
s.push(2);
s.push(3);
// print the top element
std::cout << "Top element: " << s.top() << std::endl;
// pop the top element
s.pop();
// print the new top element
std::cout << "New top element: " << s.top() << std::endl;
return 0;
}
```

The above examples demonstrate the basic functionality of a stack, but keep in mind that you can add more functionality, such as checking if the stack is empty or the size of the stack, by using the provided methods in the standard libraries of each language.

## Applications of Stacks

Stacks are used in a wide variety of algorithms and applications, some of the most common include:

**Memory management:**The operating system uses a stack to keep track of the memory allocated to different processes. This allows the system to free up memory when a process is no longer in use.**Function calls:**The call stack is used to keep track of the sequence of function calls in a program. Each time a function is called, a new frame is pushed onto the stack, and each time a function returns, the corresponding frame is popped from the stack.**Expression evaluation:**Stacks are used to evaluate expressions in reverse Polish notation (postfix notation) and infix to postfix/prefix conversion.**Syntax parsing:**Stacks are used to check for matching delimiters in a programming language, such as matching parentheses in a math expression.**Undo/Redo functionality:**Stacks are used in text editors and other software to implement undo and redo functionality. Each time an action is taken, the previous state is pushed onto the stack, and each undo action pops a state from the stack.**Back functionality in web browsers:**Stacks are used to keep track of the pages visited by the user, allowing them to go back to the previous page by popping the top page of the stack.**Graph traversal:**Stacks are used to implement depth-first search algorithms in graph traversal.**Compilers and Interpreters:**Stacks are used in many compilers and interpreters for parsing and evaluating expressions and symbols.**Garbage collection:**Stacks are used in garbage collection algorithms to keep track of objects that are still in use.

These are just a few examples of how stacks are used in various applications and algorithms. The versatility and efficiency of stacks make them a valuable tool in computer science and programming.

## Time and Space Complexity of Stack

The **time complexity of the basic stack operations (push, pop, peek) is usually O(1)** or constant time. This means that the time it takes to perform these operations is not affected by the number of items in the stack.

**The space complexity of a stack is O(n)** where n is the number of items in the stack. This means that the amount of memory used by the stack increases linearly with the number of items stored in it.

It's worth noting that the space complexity could be affected by the choice of implementation, for example, if the stack is implemented using an array, the space complexity will be O(n) where as it could be O(1) if implemented using linked list. Also, in some cases, the space complexity could be affected by the size of the data type stored in the stack.

In summary, the time complexity of basic stack operations is O(1) and the space complexity is O(n). These are considered optimal complexities for a stack data structure and make it a valuable tool in various algorithms and applications.

## Advantages and Disadvantages of Stack

**Advantages of Stacks:**

**Efficient operations:**The basic stack operations (push, pop, peek) have a time complexity of O(1), making them very efficient for large data sets.**Memory Efficient:**Stacks have a space complexity of O(n), where n is the number of items in the stack, which makes them memory-efficient.**LIFO principle:**The Last In First Out (LIFO) principle of stacks makes them useful in situations where the most recent item added is the most important, such as undo functionality in text editors, back functionality in web browsers, and function call tracking in a call stack.**Various Applications:**Stacks are used in many algorithms and applications, such as infix to postfix/prefix conversion, expression evaluation, and memory management.

**Disadvantages of Stacks:**

**Limited Access:**Stacks only allow access to the top element and do not allow for efficient access to any other element in the stack.**Fixed size:**Some implementations of stacks have a fixed size limit, which can be a problem if the number of items in the stack exceeds the limit.**Limited Functionality:**Stacks only provide basic functionality and do not have additional functionality such as sorting, searching or random access.**Not suitable for FIFO:**Stacks are not suitable for situations where the First In First Out (FIFO) principle is needed, such as a queue.

In conclusion, Stacks are a useful data structure that provides efficient operations and memory usage, and it's suitable for situations where the LIFO principle is needed. However, it has its limitations in terms of functionality, access and size, and it's not suitable for situations where the FIFO principle is needed. It's important to evaluate the specific requirements of the problem before choosing a stack as the solution.