diff --git a/main.cpp b/main.cpp
index dc25c6b..6499b4f 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1,119 +1,118 @@
-/* 基于智能指针实现双向链表 */
-#include <cstdio>
-#include <memory>
-
-struct Node {
-    // 这两个指针会造成什么问题？请修复
-    std::shared_ptr<Node> next;
-    std::shared_ptr<Node> prev;
-    // 如果能改成 unique_ptr 就更好了!
-
-    int value;
-
-    // 这个构造函数有什么可以改进的？
-    Node(int val) {
-        value = val;
-    }
-
-    void insert(int val) {
-        auto node = std::make_shared<Node>(val);
-        node->next = next;
-        node->prev = prev;
-        if (prev)
-            prev->next = node;
-        if (next)
-            next->prev = node;
-    }
-
-    void erase() {
-        if (prev)
-            prev->next = next;
-        if (next)
-            next->prev = prev;
-    }
-
-    ~Node() {
-        printf("~Node()\n");   // 应输出多少次？为什么少了？
-    }
-};
-
-struct List {
-    std::shared_ptr<Node> head;
-
-    List() = default;
-
-    List(List const &other) {
-        printf("List 被拷贝！\n");
-        head = other.head;  // 这是浅拷贝！
-        // 请实现拷贝构造函数为 **深拷贝**
-    }
-
-    List &operator=(List const &) = delete;  // 为什么删除拷贝赋值函数也不出错？
-
-    List(List &&) = default;
-    List &operator=(List &&) = default;
-
-    Node *front() const {
-        return head.get();
-    }
-
-    int pop_front() {
-        int ret = head->value;
-        head = head->next;
-        return ret;
-    }
-
-    void push_front(int value) {
-        auto node = std::make_shared<Node>(value);
-        node->next = head;
-        if (head)
-            head->prev = node;
-        head = node;
-    }
-
-    Node *at(size_t index) const {
-        auto curr = front();
-        for (size_t i = 0; i < index; i++) {
-            curr = curr->next.get();
-        }
-        return curr;
-    }
-};
-
-void print(List lst) {  // 有什么值得改进的？
-    printf("[");
-    for (auto curr = lst.front(); curr; curr = curr->next.get()) {
-        printf(" %d", curr->value);
-    }
-    printf(" ]\n");
-}
-
-int main() {
-    List a;
-
-    a.push_front(7);
-    a.push_front(5);
-    a.push_front(8);
-    a.push_front(2);
-    a.push_front(9);
-    a.push_front(4);
-    a.push_front(1);
-
-    print(a);   // [ 1 4 9 2 8 5 7 ]
-
-    a.at(2)->erase();
-
-    print(a);   // [ 1 4 2 8 5 7 ]
-
-    List b = a;
-
-    a.at(3)->erase();
-
-    print(a);   // [ 1 4 2 5 7 ]
-    print(b);   // [ 1 4 2 8 5 7 ]
-
-    b = {};
-    a = {};
-
-    return 0;
-}
+/* 基于智能指针实现双向链表 */
+#include <cstdio>
+#include <memory>
+
+struct Node {
+    // 这两个指针会造成什么问题？请修复
+    std::shared_ptr<Node> next;
+    std::shared_ptr<Node> prev;
+    // 如果能改成 unique_ptr 就更好了!
+
+    int value;
+
+    // 这个构造函数有什么可以改进的？
+    Node(int val):value(val) {
+    }
+
+    void insert(int val) {
+        auto node = std::make_shared<Node>(val);
+        node->next = next;
+        node->prev = prev;
+        if (prev)
+            prev->next = node;
+        if (next)
+            next->prev = node;
+    }
+
+    void erase() {
+        if (prev)
+            prev->next = next;
+        if (next)
+            next->prev = prev;
+    }
+
+    ~Node() {
+        printf("~Node()\n");   // 应输出多少次？为什么少了？
+    }
+};
+
+struct List {
+    std::shared_ptr<Node> head;
+
+    List() = default;
+
+    List(List const &other) {
+        printf("List 被拷贝！\n");
+        head = other.head;  // 这是浅拷贝！
+        // 请实现拷贝构造函数为 **深拷贝**
+    }
+
+    List &operator=(List const &) = delete;  // 为什么删除拷贝赋值函数也不出错？
+
+    List(List &&) = default;
+    List &operator=(List &&) = default;
+
+    Node *front() const {
+        return head.get();
+    }
+
+    int pop_front() {
+        int ret = head->value;
+        head = head->next;
+        return ret;
+    }
+
+    void push_front(int value) {
+        auto node = std::make_shared<Node>(value);
+        node->next = head;
+        if (head)
+            head->prev = node;
+        head = node;
+    }
+
+    Node *at(size_t index) const {
+        auto curr = front();
+        for (size_t i = 0; i < index; i++) {
+            curr = curr->next.get();
+        }
+        return curr;
+    }
+};
+
+void print(List lst) {  // 有什么值得改进的？
+    printf("[");
+    for (auto curr = lst.front(); curr; curr = curr->next.get()) {
+        printf(" %d", curr->value);
+    }
+    printf(" ]\n");
+}
+
+int main() {
+    List a;
+
+    a.push_front(7);
+    a.push_front(5);
+    a.push_front(8);
+    a.push_front(2);
+    a.push_front(9);
+    a.push_front(4);
+    a.push_front(1);
+
+    print(a);   // [ 1 4 9 2 8 5 7 ]
+
+    a.at(2)->erase();
+
+    print(a);   // [ 1 4 2 8 5 7 ]
+
+    List b = a;
+
+    a.at(3)->erase();
+
+    print(a);   // [ 1 4 2 5 7 ]
+    print(b);   // [ 1 4 2 8 5 7 ]
+
+    b = {};
+    a = {};
+
+    return 0;
+}