樣例

給出 1->1->2->null，返回 1->2->null

給出 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;
    }