diff --git a/main.cpp b/main.cpp
index dc25c6b..1c5b231 100644
--- a/main.cpp
+++ b/main.cpp
@@ -2,93 +2,124 @@
 #include <cstdio>
 #include <memory>
 
-struct Node {
+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;
+    std::unique_ptr<Node> next;
+    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;
+    Node(int val) : value(val), prev(nullptr), next(nullptr)
+    {
     }
 
-    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 insert(int val)
+    {
+        auto node = std::make_unique<Node>(val);
+        node->next = std::move(next);
+        node->prev = this;
+        if (node->next)
+            node->next->prev = node.get();
+        next = std::move(node);
     }
 
-    void erase() {
-        if (prev)
-            prev->next = next;
+    void erase()
+    {
+
         if (next)
             next->prev = prev;
+        if (prev)
+            prev->next = std::move(next);
+        prev = nullptr;
     }
 
-    ~Node() {
-        printf("~Node()\n");   // 应输出多少次？为什么少了？
+    ~Node()
+    {
+        printf("~Node()\n"); // 应输出多少次？为什么少了？
     }
 };
 
-struct List {
-    std::shared_ptr<Node> head;
+struct List
+{
+    std::unique_ptr<Node> head;
 
     List() = default;
 
-    List(List const &other) {
+    List(List const &other)
+    {
         printf("List 被拷贝！\n");
-        head = other.head;  // 这是浅拷贝！
+        // head = other.head; // 这是浅拷贝！
         // 请实现拷贝构造函数为 **深拷贝**
+        if (other.head)
+        {
+            head = std::make_unique<Node>(other.head->value);
+            Node *curr = head.get();
+            Node *nextCurr = other.head->next.get();
+            while (nextCurr)
+            {
+                auto newNode = std::make_unique<Node>(nextCurr->value);
+                curr->next = std::move(newNode);
+                curr->next->prev = curr;
+                curr = curr->next.get();
+                nextCurr = nextCurr->next.get();
+            }
+        }
     }
 
-    List &operator=(List const &) = delete;  // 为什么删除拷贝赋值函数也不出错？
+    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 {
+    Node *front() const
+    {
         return head.get();
     }
 
-    int pop_front() {
+    int pop_front()
+    {
         int ret = head->value;
-        head = head->next;
+        head = std::move(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;
+    void push_front(int value)
+    {
+        auto node = std::make_unique<Node>(value);
+        node->next = std::move(head);
+        if (node->next)
+            node->next->prev = node.get();
+        head = std::move(node);
     }
 
-    Node *at(size_t index) const {
+    Node *at(size_t index) const
+    {
         auto curr = front();
-        for (size_t i = 0; i < index; i++) {
+        for (size_t i = 0; i < index; i++)
+        {
+            if (!curr)
+                return nullptr;
             curr = curr->next.get();
         }
         return curr;
     }
 };
 
-void print(List lst) {  // 有什么值得改进的？
+void print(const List &lst)
+{ // 有什么值得改进的？
     printf("[");
-    for (auto curr = lst.front(); curr; curr = curr->next.get()) {
+    for (auto curr = lst.front(); curr; curr = curr->next.get())
+    {
         printf(" %d", curr->value);
     }
     printf(" ]\n");
 }
 
-int main() {
+int main()
+{
     List a;
 
     a.push_front(7);
@@ -99,18 +130,18 @@ int main() {
     a.push_front(4);
     a.push_front(1);
 
-    print(a);   // [ 1 4 9 2 8 5 7 ]
+    print(a); // [ 1 4 9 2 8 5 7 ]
 
     a.at(2)->erase();
 
-    print(a);   // [ 1 4 2 8 5 7 ]
+    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 ]
+    print(a); // [ 1 4 2 5 7 ]
+    print(b); // [ 1 4 2 8 5 7 ]
 
     b = {};
     a = {};