* Definition for singly-linked list.
* public class ListNode {
* ListNode(int x) { val = x; }
public ListNode deleteDuplicates(ListNode head) {
ListNode dummy = new ListNode(0);
ListNode preNode = dummy, curNode = head;
while (curNode != null) {
while (curNode.next != null && curNode.val == curNode.next.val) {
if (preNode.next != curNode) {
preNode.next = curNode.next;