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324. Stack Data Structure – Introduction and Implementation

What is Stack??

  • Stack is an abstract data type (ADT) and very useful in programming.
  • In computer science, a stack is an abstract data type that serves as a collection of elements.
  • Majorly all the operations are done at only one end of the stack which is top of the stack.
  • The order in which elements in stack are stored is with LIFO (Last-in-First-Out) manner. Means the element inserted last will be removed first.

See the diagram below for more understanding.

Stack Operations

Operations on Stack and How to Implement:

Java already has a built-in class for Stack. Click here to read about it.

Ways to implement Stack

  1. Implement stack using Array (discussed in this article)
  2. Implement stack using LinkedList


Initialize a topIndex = -1;

  • For push() operation- increment topIndex by 1 and add element at this index in array. Time- O(1)
  • For pop() operation- remove element from the topIndex, decrement topIndex by. Time- O(1)
  • For peek() operation- return the element from the topIndex. Time- O(1)
  • For isEmpty() operation- check if topIndex<0 then return true else false. Time- O(1)
  • For getSize() operation- return the topindex+1. Time- O(1)


Is Stack Empty: true
Inserting 1 into Stack.
Inserting 2 into Stack.
Inserting 3 into Stack.
Inserting 4 into Stack.
Stack overflow, ...cannot insert new element
Stack (top-->bottom): 4 3 2 1
Popping from Stack.
Popped Element: 4
Popping from Stack.
Popped Element: 3
Stack (top-->bottom): 2 1
Inserting 6 into Stack.
Popping from Stack.
Peeking Element: 6
Stack (top-->bottom): 6 2 1
Is Stack Empty: false
Stack Size: 3