In this blog, we are going to learn about stack and its implementation with a detailed example.

Stack

• A stack is a list of elements in which an element may be inserted or deleted only at one end, called the “TOP” of the Stack.
• Stack is a LIFO (Last In First Out) data structure.
• Elements are removed from a stack in the reverse order of that in which they were inserted into the stack.
• Stack is an Abstract Data Type.

Stack Operations

• LIFO - Last in, First out.
• PUSH - Adds an item as the top item on the stack.
• POP - Removes the top item on the stack.
• PEEK - Gets the top item on the stack without popping it.

Examples: Stack of Dishes, Stack of Books, packet of Biscuit etc.

Note: AVAIL List is also implemented using STACK.

Stack Time Complexity

• O(1) for push, pop and peek, when using a linked list.
• If you use any array then push is O(n), because the array may have to be resized.
• If you know the maximum number of items that are on the stack, an array could be a good choice.
• If memory has limitations array is a good choice.
• Linked List is Ideal.

import java.util.Stack;

public class BehindJava {
	public static void main(String[] args) {

		Stack<Integer> stk = new Stack<>();
		stk.push(10);
		stk.push(9);
		stk.push(8);
		stk.push(7);
		stk.push(6);
		stk.push(5);
		System.out.println(stk);
		stk.pop();
		System.out.println(stk);
		stk.peek();
		System.out.println(stk);
		System.out.println(stk.peek());
	}
}

Output:

[10, 9, 8, 7, 6, 5]
[10, 9, 8, 7, 6]
[10, 9, 8, 7, 6]
6