Min Value Stack Get in O(1)

min-value-stack.js

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

class ItemStack {
    constructor(){
        this.first = null;
        this.last = null;
        this.length = 0;
        this.minStack = new MinStack();
    }

    push(val){
        let newNode = new Node(val)
        if (this.length === 0){
            this.first = newNode;
            this.last = newNode;
            
        }else{
           let temp = this.first;
           this.first = newNode;
           this.first.next= temp;
        }
        this.minStack.push(val)
        this.length++;     
        return this.length;
    }

    pop(){
        if (this.length === 0) return null;

        let temp = this.first;

        if (this.length === 1){
            this.last = null;
        }        
        this.first = this.first.next;    
        this.length--;
        
        if(temp.val === this.minStack.first.val){
            this.minStack.pop()
        }

        return temp;
    }

    min(){
        if(this.length === 0) return undefined;
        else return this.minStack.first.val;
    }

}

class MinStack {

    constructor(){
        this.first = null;
        this.last = null;
        this.length = 0;
    }

    push(val) {
        let newNode = new Node(val);
        if (this.length === 0) {
            this.first = newNode;
            this.last = newNode;
            this.length++;
        }else {
            if (val < this.first.val) {
                let temp = this.first;
                this.first = newNode;
                this.first.next= temp;
                this.length++;
            }
        }

        return this.length;
    }

    pop() {
        
        let temp = this.first;

        if (this.length === 1){
            this.last = null;
        }        
        this.first = this.first.next;        
        this.length--;
        return temp;
    }

}

let itemStack = new ItemStack();

itemStack.push(3);
itemStack.push(4);
itemStack.push(1);
console.log(itemStack.min());
itemStack.pop();
console.log(itemStack.min());
itemStack.pop();
console.log(itemStack.min());
itemStack.pop();
console.log(itemStack.min());