[LeedCode]Remove Duplicates from Sorted List II

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode pre = new ListNode(0);
        pre.next = head;
        head = pre;
        ListNode p = pre.next;
        while (p != null) {
            if (p.next != null && p.val == p.next.val) {
                ListNode q = p;
                while (q != null && q.val == p.val) {
                    q = q.next;
                }
                pre.next = q;
                p = q;
            } else {
                pre = p;
                p = p.next;
            }
        }
        return head.next;
    }
}