Leetcode-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.

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (head==null || head.next==null) return head;
        ListNode preHead = new ListNode(-1);
        preHead.next = head;
        ListNode cur = head;
        ListNode pre = preHead;

        //NOTE:We need to be every carefull about the condition,
        //because if the last node be deleted, the cur is null in the next step.
        //however, if the node before the last node is deleted, the cur is the last node in the next step,
        //we then need to stop also, otherwise cur.next.val is invalid.
        while (cur!=null && cur.next!=null){
            if (cur.val==cur.next.val){
                while (cur.val==cur.next.val){
                    cur.next = cur.next.next;
                    if (cur.next==null)
                        break;
                }
                pre.next = cur.next;                                
                cur = pre.next;
            } else {
                pre = cur;
                cur = cur.next;
                if (cur.next==null)
                    break;
            }
        }

        return preHead.next;            
    }
}