A stack is an Abstract Data Type (ADT), commonly used in most programming languages. It is named stack as it behaves like a real-world stack, for example – a deck of cards or a pile of plates, etc.
A real-world stack allows operations at one end only. For example, we can place or remove a card or plate from the top of the stack only. Likewise, Stack ADT allows all data operations at one end only. At any given time, we can only access the top element of a stack.
This feature makes it LIFO data structure. LIFO stands for Last-in-first-out. Here, the element which is placed (inserted or added) last, is accessed first. In stack terminology, insertion operation is called PUSH operation and removal operation is called POP operation.
Conceptually, a stack is simple: a data structure that allows adding and removing elements in a particular order. Every time an element is added, it goes on the top of the stack; the only element that can be removed is the element that was at the top of the stack. Consequently, a stack is said to have "first in last out" behavior (or "last in, first out"). The first item added to a stack will be the last item removed from a stack.
Stack operations may involve initializing the stack, using it and then de-initializing it. Apart from these basic stuffs, a stack is used for the following two primary operations −
The process of putting a new data element onto stack is known as a Push Operation.
There is a pointer generally known as Top which tracks the last inserted position or the top of the Stack. In every insertion and deletion in the Stack will update this Top as well.
To insert a new element to the Stack, first check if the stack is full (for fixed size Stack). If the stack is not full then insert the element at the top of the Stack and update the Top.
Accessing the content while removing it from the stack, is known as a Pop Operation.
In Pop operation, if the Stack is not empty then the last inserted element is removed and the Top points to the next element.
To use a stack efficiently, we need to check the status of stack as well. For the same purpose, the following functionality are available too −
- Peek − get the top data element of the Stack, without removing it.
- isEmpty − check if Stack is empty.
In JavaScript, Stack can be easily implemented by an Array. JavaScript Arrays have the built-in methods called 'push' and 'pop', which do exactly the same operation as Stack.
- Insertion - O(1)
- Deletion - O(1)
- Access - O(n)
- Search - O(n)