reverse_list.rs

struct ListNode<'a, T> where T: PartialOrd + 'a {
    val: &'a T,
    next: Option<Box<ListNode<'a, T>>>,
}

fn reverse_list<'a, T>(head: Option<Box<ListNode<'a, T>>>) -> Option<Box<ListNode<'a, T>>>
    where T: 'a + PartialOrd {
        if let Some(mut p) = head {
            let mut tail = None;
            loop {
                let p_next = p.next.take();
                p.next = tail;
                tail = Some(p);
                match p_next {
                    None => break,
                    _    => p = p_next.unwrap(),
                };
            };
            tail
        } else {
            None
        }
}