theprimeagen / fem-algos Goto Github PK

D:

First of all, this is a great course (still working through it...).

One thing that really tripped me off is the Queue.enque() method:

type Node<T> = {
    value: T;
    next?: Node<T>;
}

export default class Queue<T> {
    public length: number;
    private head?: Node<T>;
    private tail?: Node<T>;

    constructor() {
        this.head = this.tail = undefined;
        this.length = 0;
    }

    enqueue(item: T): void {
        const newNode = {value: item} as Node<T>;
        this.length++;
        if (!this.tail) {
            this.tail = this.head = newNode;
            return;
        }
        this.tail.next = newNode;
        this.tail = newNode;        
    }

    deque(): T | undefined {
        if (!this.head) {
            return undefined;
        }
        this.length--;

        const head = this.head;
        this.head = this.head.next;

        if (this.length === 0) {
            this.tail = undefined;
        }

        return head.value;
    }
    peek(): T | undefined {
        return this.head?.value;
    }   
}   

The confusing part is these two lines:

this.tail.next = newNode;
this.tail = newNode; 

This doesn't have the effect as explained in the video lecture (first point the element at this.tail.next to the new node and then set this.tail to the same node). The second line - this.tail = newNode - has as the effect that this.tail.next is always undefined (upon returning from the method).

But in one very special case, when this.tail and this.head point to the same Node object (the line: this.tail = this.head = newNode; in the if-condition) it does affect the this.head.next.

I still have trouble comprehending this implementation w/o using a debugger. Has someone found a less obscrure implementation?

In the course @ThePrimeagen you have explained that the while condition should be lo < hi and not lo <= hi .

But it is wrong, this condition will fail for the edge case where the needle happens to be the last element.

Infact, a guy does ask you the question of why it is not lesser than or equal to and you reply saying it is due to the inclusiveness of the lower bound and exclusiveness of the upper bound and this doesn't make any sense, for me at least.

The class link at 4:50

I would have asked it in the discussion section but it has not been enabled for this repo. Please do so, or just share some link to the community groups for this course.

Really appreciate it,
thanks for your service 🫡

Link on the website points to

https://frontendmasters.com/course/algorithms/

but it should be

https://frontendmasters.com/courses/algorithms/

In your lesson on queues you mention that it is mind-boggling that deque is spelled this way, but as a method to remove an item from a queue, it actually should be spelled dequeue as you surmised.

Deque would be pronounced "deck" and refers to a double-ended queue data type.

https://en.wikipedia.org/wiki/Double-ended_queue