样例
给出 1->1->2->null,返回 1->2->null
给出 1->1->2->3->3->null,返回 1->2->3->null
public static ListNode deleteDuplicates(ListNode head) {
// write your code here
ListNode p = head;
while (p != null) {
ListNode q = p.next;
while (q != null && q.val == p.val) {
q = q.next;
}
p.next = q;
p = p.next;
}
return head;
}