STACKS

OBJECTIVES

  • Define what a stack is
  • Understand use cases for a stack
  • Implement operations on a stack data structure

WHAT IS A STACK?

A LIFO data structure!

The last element added to the stack will be the first element removed from the stack

HOW IS IT USED?

Think about a stack of plates, or a stack of markers, or a stack of....anything.

As you pile it up the last thing (or the topmost thing) is what gets removed first.

How we'll visualize a stack

A series of nodes!

Let's see this in action!

10

2

22

7

last

first

size = 4

WE'VE SEEN

THIS BEFORE

The Call Stack!

WHERE STACKS ARE USED

  • Managing function invocations
  • Undo / Redo
  • Routing (the history object) is treated like a stack!

THERE IS MORE THAN ONE WAY OF IMPLEMENTING A STACK

ARRAY IMPLEMENTATION

LINKED LIST IMPLEMENTATION

A STACK CLASS

class Stack {
    constructor(){
        this.first = null;
        this.last = null;
        this.size = 0;
    }
}
class Node {
    constructor(value){
        this.value = value;
        this.next = null;
    }
}

PUSHING

Add a value to the top of the stack!

PUSHING PSEUDOCODE

  • The function should accept a value
  • Create a new node with that value
  • If there are no nodes in the stack, set the first and last property to be the newly created node 
  • If there is at least one node, create a variable that stores the current first property on the stack
  • Reset the first property to be the newly created node
  • Set the next property on the node to be the previously created variable
  • Increment the size of the stack by 1

YOUR

TURN

POP

Remove a value from the top of the stack!

POP PSEUDOCODE

  • If there are no nodes in the stack, return null
  • Create a temporary variable to store the first property on the stack
  • If there is only 1 node, set the first and last property to be null
  • If there is more than one node, set the first property to be the next property on the current first
  • Decrement the size by 1
  • Return the value of the node removed

YOUR

TURN

BIG O of STACKS

Insertion -   O(1)

Removal -   O(1)

Searching -   O(N)

Access -   O(N)

RECAP

  • Stacks are a LIFO data structure where the last value in is always the first one out.
  • Stacks are used to handle function invocations (the call stack), for operations like undo/redo, and for routing (remember pages you have visited and go back/forward) and much more!
  • They are not a built in data structure in JavaScript, but are relatively simple to implement
  • Insert and remove are both O(1)