class QueueStack {
    constructor() {
        this.q = [];
    }
    push(val) {
        let size = this.q.length;
        this.q.push(val);
        while (size--) {
            this.q.push(this.q.shift());
        }
    }
    pop() {
        if (!this.q.length) return null;
        return this.q.shift();
    }
    peek() {
        if (!this.q.length) return null;
        return this.q[0];
    }

    isEmpty() {
        return this.q.length === 0
    }
}