Method popFirst

chapter03/util/LinkedList.js:54–62

()

  }

  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;