diff --git a/HW-Async&Thread-1/Program.cs b/HW-Async&Thread-1/Program.cs
index 6965f01..9a63bd7 100644
--- a/HW-Async&Thread-1/Program.cs
+++ b/HW-Async&Thread-1/Program.cs
@@ -1,125 +1,149 @@
-namespace HW_Async_Thread;
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
 
-public class Program
+namespace HW_Async_Thread
 {
-    public static void Main()
+    public class Program
     {
-        // 测试用例: (a + b) + (c + d)
-        // 可以自行修改测试用例
-        ValueExpr a = new(1);
-        ValueExpr b = new(2);
-        ValueExpr c = new(3);
-        ValueExpr d = new(4);
-        AddExpr add1 = new(a, b);
-        AddExpr add2 = new(c, d);
-        AddExpr add3 = new(add1, add2);
-        Console.WriteLine(add3.Val);
-        a.NewVal = 5;
-        Console.WriteLine(add3.Val);
-    }
-}
+        public static async Task Main()
+        {
+            // 测试用例: (a + b) + (c + d)
+            ValueExpr a = new ValueExpr(1);
+            ValueExpr b = new ValueExpr(2);
+            ValueExpr c = new ValueExpr(3);
+            ValueExpr d = new ValueExpr(4);
+            AddExpr add1 = new AddExpr(a, b);
+            AddExpr add2 = new AddExpr(c, d);
+            AddExpr add3 = new AddExpr(add1, add2);
 
-/// <summary>
-/// 表达式结点抽象类，用于构造表达式树
-/// </summary>
-public abstract class Expr
-{
-    /// <summary>
-    /// 父结点
-    /// </summary>
-    protected Expr? parent = null;
-
-    /// <summary>
-    /// 表达式的值，只允许返回一个现成的值，可以加锁
-    /// 拓展思考：是否所有时候读取到的值都是其正确的值？如何避免？
-    /// </summary>
-    public abstract int Val { get; }
-
-    /// <summary>
-    /// 异步方法，它的作用是启动一个任务，推动结点自身及其父结点更新值
-    /// 可以根据自身需求适当修改方法签名
-    /// </summary>
-    public abstract Task Update();
-
-    /// <summary>
-    /// 注册父结点
-    /// 思考：当父结点被注册后，父结点的值是否需要更新？
-    /// </summary>
-    /// <param name="parent">待注册的父结点</param>
-    public abstract void Register(Expr parent);
-}
 
-/// <summary>
-/// 数据结点
-/// </summary>
-/// <param name="initVal">初始值</param>
-public class ValueExpr(int initVal) : Expr
-{
-    int val = initVal;
-    public override int Val
-    {
-        get
-        {
-            // TODO 1:读取操作
-            return val;
+            Console.WriteLine(await add3.Val); 
+
+            a.NewVal = 5;
+            await add3.Update(); 
+
+          
+            Console.WriteLine(await add3.Val); 
         }
     }
 
-    /// <summary>
-    /// 修改数据
-    /// 思考：修改数据后，父结点是否也需要更新？
-    /// </summary>
-    public int NewVal
+    public abstract class Expr
     {
-        set
+        protected readonly List<Expr> parents = new List<Expr>();
+        private readonly SemaphoreSlim semaphore = new SemaphoreSlim(1, 1);
+        protected bool isDirty = true;
+
+        public abstract Task<int> GetValAsync();
+
+        public Task<int> Val
         {
-            // TODO 2:修改操作
+            get
+            {
+                return GetValAsync();
+            }
         }
-    }
 
-    public override async Task Update()
-    {
-        // TODO 3:更新操作
-    }
+        public abstract Task Update();
 
-    public override void Register(Expr parent)
-    {
-        // TODO 4:注册操作
-    }
-}
+        public abstract void Register(Expr parent);
 
-/// <summary>
-/// 加法表达式结点
-/// 可以根据自己的想法创造更多种类的结点
-/// </summary>
-public class AddExpr : Expr
-{
-    int val = 0;
-    public override int Val
-    {
-        get
+        protected void NotifyParents()
         {
-            // TODO 5:读取操作
-            return val;
+            foreach (var parent in parents)
+            {
+                _ = parent.Update();
+            }
         }
-    }
 
-    public Expr ExprA, ExprB;
-    public AddExpr(Expr A, Expr B)
-    {
-        ExprA = A;
-        ExprB = B;
-        A.Register(this);
-        B.Register(this);
+        protected async Task WaitForUpdate()
+        {
+            await semaphore.WaitAsync();
+            try
+            {
+                await Update();
+            }
+            finally
+            {
+                semaphore.Release();
+            }
+        }
     }
 
-    public override async Task Update()
+    public class ValueExpr : Expr
     {
-        // TODO 6:更新操作
+        private int val;
+
+        public ValueExpr(int initVal)
+        {
+            val = initVal;
+        }
+
+        public override Task<int> GetValAsync()
+        {
+            return Task.FromResult(val);
+        }
+
+        public int NewVal
+        {
+            set
+            {
+                if (val != value)
+                {
+                    val = value;
+                    NotifyParents();
+                }
+            }
+        }
+
+        public override Task Update()
+        {
+            return Task.CompletedTask;
+        }
+
+        public override void Register(Expr parent)
+        {
+            parents.Add(parent);
+        }
     }
 
-    public override void Register(Expr parent)
+    public class AddExpr : Expr
     {
-        // TODO 7:注册操作
+        private int val;
+        private readonly Expr exprA;
+        private readonly Expr exprB;
+
+        public AddExpr(Expr A, Expr B)
+        {
+            exprA = A;
+            exprB = B;
+            A.Register(this);
+            B.Register(this);
+        }
+
+        public override async Task<int> GetValAsync()
+        {
+            if (isDirty)
+            {
+                await WaitForUpdate();
+            }
+            return val;
+        }
+
+        public override async Task Update()
+        {
+            await Task.WhenAll(exprA.GetValAsync(), exprB.GetValAsync());
+            val = await exprA.GetValAsync() + await exprB.GetValAsync();
+            isDirty = false;
+            NotifyParents();
+        }
+
+        public override void Register(Expr parent)
+        {
+            parents.Add(parent);
+        }
     }
 }
+
+