Stack.js

class Node {
    constructor(value) {
        this.value = value;
        this.next = null;
    }
}


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

    push(val){
        let newNode = new Node(val);

        if (!this.first) {
            this.first = newNode;
            this.last = newNode;
        }else{
            let temp = this.first;
            this.first = newNode;
            this.first.next = temp;
        }
        return ++this.size;
    }
    pop(){
        if (!this.first) {
            return null;
        }
        let temp = this.first;
        if (this.first === this.last) {
            this.last = null;
        }
        this.first = this.first.next;
        this.size--;
        return temp.value;

    }
    
}