Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

思路：

1. 与 Remove Duplicates from Sorted List 不同， 这里是要去除所有重复过得节点。

2. 这题很难