Remove Duplicates from Sorted List II (Java)

LeetCode

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.

Solution: 

The logic is pretty straight, however, the corner cases need pay more attention.
check the comments in the code.