Leetcode-Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->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;
        
        while (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;
                }
            } else {
                pre = cur;
                cur = cur.next;
            }
        }

        return preHead.next;           
        
    }
}