text: /**
   * @param {ListNode} head
   * @return {ListNode}
   */
  var deleteDuplicates = function (head) {
      if (!head) {
          return head
      }
      let current = head
      while (current.next) {
          if (current.val == current.next.val) {
              current.next = current.next.next
          } else {
              current = current.next
          }
      }
      return head
  };