Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

Example 1:

Input: 1->2->3->3->4->4->5
Output: 1->2->5
Example 2:

Input: 1->1->1->2->3
Output: 2->3
遞迴和迭代：

public ListNode deleteDuplicates(ListNode head) {
        if(head == null || head.next == null) return head;
        int val = head.val;
        ListNode p = head.next;
        if(head.val != p.val){
            head.next = deleteDuplicates(p);
            return head;
        }
        else{
            while(p != null && val == p.val) p = p.next;
            return deleteDuplicates(p);
        }
    }

public ListNode deleteDuplicates(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode pre = dummy;
        ListNode cur = head;
        while(cur != null){
            while(cur.next != null && cur.next.val == cur.val){
                cur = cur.next;
            }
            if(pre.next == cur){
                pre = pre.next;
                cur = cur.next;
            }else{
                pre.next = cur.next;
                cur = cur.next;
            }
        }
        return dummy.next;
    }