新增章节：[deque-基本操作的对称镜像]

src/SUMMARY.md

@@ -170,6 +170,7 @@
     - [不咋样的双端队列](too-many-lists/deque/intro.md)
       - [数据布局和基本操作](too-many-lists/deque/layout.md)
       - [Peek](too-many-lists/deque/peek.md)
+      - [基本操作的对称镜像](too-many-lists/deque/symmetric.md)
 - [易混淆概念解析](confonding/intro.md)
   - [切片和切片引用](confonding/slice.md)
   - [Eq 和 PartialEq](confonding/eq.md)

src/too-many-lists/deque/symmetric.md

@@ -0,0 +1,161 @@
+# 基本操作的对称镜像
+之前我们仅实现了头部的 `push`、`pop` ，现在来补全一下，大自然的对称之美咱的双向链表也不能少了。
+
+```rust
+tail <-> head
+next <-> prev
+front -> back
+```
+
+需要注意的是，这里还新增了 `mut` 类型的 peek:
+```rust
+use std::cell::{Ref, RefCell, RefMut};
+
+//..
+
+pub fn push_back(&mut self, elem: T) {
+    let new_tail = Node::new(elem);
+    match self.tail.take() {
+        Some(old_tail) => {
+            old_tail.borrow_mut().next = Some(new_tail.clone());
+            new_tail.borrow_mut().prev = Some(old_tail);
+            self.tail = Some(new_tail);
+        }
+        None => {
+            self.head = Some(new_tail.clone());
+            self.tail = Some(new_tail);
+        }
+    }
+}
+
+pub fn pop_back(&mut self) -> Option<T> {
+    self.tail.take().map(|old_tail| {
+        match old_tail.borrow_mut().prev.take() {
+            Some(new_tail) => {
+                new_tail.borrow_mut().next.take();
+                self.tail = Some(new_tail);
+            }
+            None => {
+                self.head.take();
+            }
+        }
+        Rc::try_unwrap(old_tail).ok().unwrap().into_inner().elem
+    })
+}
+
+pub fn peek_back(&self) -> Option<Ref<T>> {
+    self.tail.as_ref().map(|node| {
+        Ref::map(node.borrow(), |node| &node.elem)
+    })
+}
+
+pub fn peek_back_mut(&mut self) -> Option<RefMut<T>> {
+    self.tail.as_ref().map(|node| {
+        RefMut::map(node.borrow_mut(), |node| &mut node.elem)
+    })
+}
+
+pub fn peek_front_mut(&mut self) -> Option<RefMut<T>> {
+    self.head.as_ref().map(|node| {
+        RefMut::map(node.borrow_mut(), |node| &mut node.elem)
+    })
+}
+```
+
+再更新测试用例:
+```rust
+#[test]
+fn basics() {
+    let mut list = List::new();
+
+    // Check empty list behaves right
+    assert_eq!(list.pop_front(), None);
+
+    // Populate list
+    list.push_front(1);
+    list.push_front(2);
+    list.push_front(3);
+
+    // Check normal removal
+    assert_eq!(list.pop_front(), Some(3));
+    assert_eq!(list.pop_front(), Some(2));
+
+    // Push some more just to make sure nothing's corrupted
+    list.push_front(4);
+    list.push_front(5);
+
+    // Check normal removal
+    assert_eq!(list.pop_front(), Some(5));
+    assert_eq!(list.pop_front(), Some(4));
+
+    // Check exhaustion
+    assert_eq!(list.pop_front(), Some(1));
+    assert_eq!(list.pop_front(), None);
+
+    // ---- back -----
+
+    // Check empty list behaves right
+    assert_eq!(list.pop_back(), None);
+
+    // Populate list
+    list.push_back(1);
+    list.push_back(2);
+    list.push_back(3);
+
+    // Check normal removal
+    assert_eq!(list.pop_back(), Some(3));
+    assert_eq!(list.pop_back(), Some(2));
+
+    // Push some more just to make sure nothing's corrupted
+    list.push_back(4);
+    list.push_back(5);
+
+    // Check normal removal
+    assert_eq!(list.pop_back(), Some(5));
+    assert_eq!(list.pop_back(), Some(4));
+
+    // Check exhaustion
+    assert_eq!(list.pop_back(), Some(1));
+    assert_eq!(list.pop_back(), None);
+}
+
+#[test]
+fn peek() {
+    let mut list = List::new();
+    assert!(list.peek_front().is_none());
+    assert!(list.peek_back().is_none());
+    assert!(list.peek_front_mut().is_none());
+    assert!(list.peek_back_mut().is_none());
+
+    list.push_front(1); list.push_front(2); list.push_front(3);
+
+    assert_eq!(&*list.peek_front().unwrap(), &3);
+    assert_eq!(&mut *list.peek_front_mut().unwrap(), &mut 3);
+    assert_eq!(&*list.peek_back().unwrap(), &1);
+    assert_eq!(&mut *list.peek_back_mut().unwrap(), &mut 1);
+}
+```
+
+什么？你问我这里的测试用例全吗？只能说如果测试全部的组合情况，这一章节会被撑爆。至于现在，能不出错就谢天谢地了 :(
+
+```shell
+> cargo test
+
+     Running target/debug/lists-5c71138492ad4b4a
+
+running 10 tests
+test first::test::basics ... ok
+test fourth::test::basics ... ok
+test second::test::basics ... ok
+test fourth::test::peek ... ok
+test second::test::iter ... ok
+test third::test::iter ... ok
+test second::test::into_iter ... ok
+test second::test::iter_mut ... ok
+test second::test::peek ... ok
+test third::test::basics ... ok
+
+test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured
+```
+
+我想说：Ctrl CV 是最好的编程工具，大家同意吗？