text: /**
   * Problem: Remove Duplicates from Sorted List
   * Difficulty: Easy
   * Runtime: 76 ms
   * Date: 2018/04/05
   * Author: Aonan He
   */
  /**
   * Definition for singly-linked list.
   * function ListNode(val) {
   *     this.val = val;
   *     this.next = null;
   * }
   */
  /**
   * @param {ListNode} head
   * @return {ListNode}
   */
  var deleteDuplicates = function(head) {
      let pre = head
      if (!head) { return head }
      let p = pre.next
      while (p) {
          while ((p) && (pre.val === p.val)) {
              p = p.next
          }
          pre.next = p
          pre = p
          p = pre && pre.next
      }
      return head
  };
  