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输入: 1->2->3->3->4->4->5
输出: 1->2->5

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输入: 1->1->1->2->3
输出: 2->3

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impl Solution {
    pub fn delete_duplicates(head: Option<Box<ListNode>>) -> Option<Box<ListNode>> {
        let mut dummy_head = Some(Box::new(ListNode::new(0)));
        let mut ptr = &mut dummy_head;

        let mut pre_val: i32 = -1;
        let mut pre_val_init = false;
        let mut next_val: i32 = 0;
        let mut head = head;
        while let Some(mut node) = head {
            head = node.next.take();
            if head.is_some() {
                if node.val == head.as_ref().unwrap().val {
                    pre_val_init = true;
                    pre_val = node.val;
                    continue
                }
            }
            if pre_val_init && pre_val == node.val {
                continue
            }
            pre_val = node.val;
            pre_val_init = true;
            ptr.as_mut().unwrap().next = Some(node);
            ptr = &mut ptr.as_mut().unwrap().next;
        }

        dummy_head.unwrap().next

    }
}