# 链表

A linked list is also a `linear` data structure, and each element in the linked list is actually a separate object while all the objects are `linked together by the reference filed` in each element. In a `doubly linked list`, each node contains, besides the `next` node link, a second link field pointing to the `previous` node in the sequence. The two links may be called `next` and `prev`. And many modern operating systems use doubly linked lists to maintain references to active processes, threads and other dynamic objects.

__Properties__
* Indexing O(n)
* Insertion O(1)
  * Beginning O(1)
  * Middle (Indexing time+O(1))
  * End O(n)
* Deletion O(1)
  * Beginning O(1)
  * Middle (Indexing time+O(1))
  * End O(n)
* Search O(n)

```rust
use std::fmt::{self, Display, Formatter};
use std::ptr::NonNull;

struct Node<T> {
    val: T,
    next: Option<NonNull<Node<T>>>,
    prev: Option<NonNull<Node<T>>>,
}

impl<T> Node<T> {
    fn new(t: T) -> Node<T> {
        Node {
            val: t,
            prev: None,
            next: None,
        }
    }
}

pub struct LinkedList<T> {
    length: u32,
    start: Option<NonNull<Node<T>>>,
    end: Option<NonNull<Node<T>>>,
}

impl<T> Default for LinkedList<T> {
    fn default() -> Self {
        Self::new()
    }
}

impl<T> LinkedList<T> {
    pub fn new() -> Self {
        Self {
            length: 0,
            start: None,
            end: None,
        }
    }

    pub fn add(&mut self, obj: T) {
        let mut node = Box::new(Node::new(obj));
        // Since we are adding node at the end, next will always be None
        node.next = None;
        node.prev = self.end;
        // Get a pointer to node
        let node_ptr = Some(unsafe { NonNull::new_unchecked(Box::into_raw(node)) });
        match self.end {
            // This is the case of empty list
            None => self.start = node_ptr,
            Some(end_ptr) => unsafe { (*end_ptr.as_ptr()).next = node_ptr },
        }
        self.end = node_ptr;
        self.length += 1;
    }

    pub fn get(&mut self, index: i32) -> Option<&T> {
        self.get_ith_node(self.start, index)
    }

    fn get_ith_node(&mut self, node: Option<NonNull<Node<T>>>, index: i32) -> Option<&T> {
        match node {
            None => None,
            Some(next_ptr) => match index {
                0 => Some(unsafe { &(*next_ptr.as_ptr()).val }),
                _ => self.get_ith_node(unsafe { (*next_ptr.as_ptr()).next }, index - 1),
            },
        }
    }
}

impl<T> Display for LinkedList<T>
where
    T: Display,
{
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self.start {
            Some(node) => write!(f, "{}", unsafe { node.as_ref() }),
            None => Ok(()),
        }
    }
}

impl<T> Display for Node<T>
where
    T: Display,
{
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        match self.next {
            Some(node) => write!(f, "{}, {}", self.val, unsafe { node.as_ref() }),
            None => write!(f, "{}", self.val),
        }
    }
}

#[cfg(test)]
mod tests {
    use super::LinkedList;

    #[test]
    fn create_numeric_list() {
        let mut list = LinkedList::<i32>::new();
        list.add(1);
        list.add(2);
        list.add(3);
        println!("Linked List is {}", list);
        assert_eq!(3, list.length);
    }

    #[test]
    fn create_string_list() {
        let mut list_str = LinkedList::<String>::new();
        list_str.add("A".to_string());
        list_str.add("B".to_string());
        list_str.add("C".to_string());
        println!("Linked List is {}", list_str);
        assert_eq!(3, list_str.length);
    }

    #[test]
    fn get_by_index_in_numeric_list() {
        let mut list = LinkedList::<i32>::new();
        list.add(1);
        list.add(2);
        println!("Linked List is {}", list);
        let retrived_item = list.get(1);
        assert!(retrived_item.is_some());
        assert_eq!(2 as i32, *retrived_item.unwrap());
    }

    #[test]
    fn get_by_index_in_string_list() {
        let mut list_str = LinkedList::<String>::new();
        list_str.add("A".to_string());
        list_str.add("B".to_string());
        println!("Linked List is {}", list_str);
        let retrived_item = list_str.get(1);
        assert!(retrived_item.is_some());
        assert_eq!("B", *retrived_item.unwrap());
    }
}
```