Stack Data Structure (Introduction and Program)
- Difficulty Level : Easy
- Last Updated : 14 Jul, 2022
Stack is a linear data structure that follows a particular order in which the operations are performed. The order may be LIFO(Last In First Out) or FILO(First In Last Out).
This strategy states that the element that is inserted last will come out first. You can take a pile of plates kept on top of each other as a real-life example. The plate which we put last is on the top and since we remove the plate that is at the top, we can say that the plate that was put last comes out first.
Some of its main operations are: push(), pop(), top(), isEmpty(), size(), etc.
In order to make manipulations in a stack, there are certain operations provided to us. When we want to insert an element into the stack the operation is known as the push operation whereas when we want to remove an element from the stack the operation is known as the pop operation. If we try to pop from an empty stack then it is known as underflow and if we try to push an element in a stack that is already full, then it is known as overflow.
Mainly the following four basic operations are performed in the stack:
Push: Adds an item to the stack. If the stack is full, then it is said to be an Overflow condition.
Algorithm for push:
begin if stack is full return endif else increment top stack[top] assign value end else end procedure
Pop: Removes an item from the stack. The items are popped in the reversed order in which they are pushed. If the stack is empty, then it is said to be an Underflow condition.
Algorithm for pop:
begin if stack is empty return endif else store value of stack[top] decrement top return value end else end procedure
Peek or Top: Returns the top element of the stack.
Algorithm for peek:
begin return stack[top] end procedure
isEmpty: Returns true if the stack is empty, else false.
Algorithm for isEmpty:
begin if top < 1 return true else return false end procedure
How to understand a stack practically?
There are many real-life examples of a stack. Consider the simple example of plates stacked over one another in a canteen. The plate which is at the top is the first one to be removed, i.e. the plate which has been placed at the bottommost position remains in the stack for the longest period of time. So, it can be simply seen to follow the LIFO/FILO order.
Time Complexities of operations on the stack:
push(), pop(), isEmpty() and peek() all take O(1) time. We do not run any loop in any of these operations.
Types of Stacks:
- Register Stack: This type of stack is also a memory element present in the memory unit and can handle a small amount of data only. The height of the register stack is always limited as the size of the register stack is very small compared to the memory.
- Memory Stack: This type of stack can handle a large amount of memory data. The height of the memory stack is flexible as it occupies a large amount of memory data.
Applications of the stack:
- Balancing of symbols
- Infix to Postfix /Prefix conversion
- Redo-undo features at many places like editors, photoshop.
- Forward and backward features in web browsers
- Used in many algorithms like Tower of Hanoi, tree traversals, stock span problems, and histogram problems.
- Backtracking is one of the algorithm designing techniques. Some examples of backtracking are the Knight-Tour problem, N-Queen problem, find your way through a maze, and game-like chess or checkers in all these problems we dive into someway if that way is not efficient we come back to the previous state and go into some another path. To get back from a current state we need to store the previous state for that purpose we need a stack.
- In Graph Algorithms like Topological Sorting and Strongly Connected Components
- In Memory management, any modern computer uses a stack as the primary management for a running purpose. Each program that is running in a computer system has its own memory allocations
- String reversal is also another application of stack. Here one by one each character gets inserted into the stack. So the first character of the string is on the bottom of the stack and the last element of a string is on the top of the stack. After Performing the pop operations on the stack we get a string in reverse order.
There are two ways to implement a stack:
- Using array
- Using linked list
Implementing Stack using Arrays
10 pushed into stack 20 pushed into stack 30 pushed into stack 30 Popped from stack Top element is : 20 Elements present in stack : 20 10
Pros: Easy to implement. Memory is saved as pointers are not involved.
Cons: It is not dynamic. It doesn’t grow and shrink depending on needs at runtime.
Implementing Stack using Linked List:
10 pushed to stack 20 pushed to stack 30 pushed to stack 30 popped from stack Top element is 20 Elements present in stack : 20 10
Advantages of Stack:
- The linked list implementation of a stack can grow and shrink according to the needs at runtime.
- It is used in many virtual machines like JVM.
- Stacks are more secure and reliable as they do not get corrupted easily.
- Stack cleans up the objects automatically.
Disadvantages of Stack:
- Requires extra memory due to involvement of pointers.
- Random accessing is not possible in stack.
- The total of size of the stack must be defined before.
- If the stack falls outside the memory it can lead to abnormal termination.
We will cover the implementation of applications of the stack in separate posts.
Quiz: Stack Questions