Method popFirst

chapter02/util/LinkedListX.js:67–75

()

    }
  
    popFirst() {
        let first = this.head;
        if (this.head && this.head.next) {
            this.head = this.head.next;
            first.next = null;
        }
        else this.head = null;
        return first;
    }
  
    head() {
        return this.head;