diff --git a/Skeleton.csproj b/Skeleton.csproj
index 138c6c8..f7ffdab 100644
--- a/Skeleton.csproj
+++ b/Skeleton.csproj
@@ -3,7 +3,7 @@
     <PropertyGroup>
         <TargetFramework>net8.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
-        <Nullable>enable</Nullable>
+        <Nullable>disable</Nullable>
         <Version>0.0.5</Version>
         <Description>    </Description>
         <Copyright>hzhang</Copyright>
diff --git a/global.json b/global.json
index 1bcf6c0..2ddda36 100644
--- a/global.json
+++ b/global.json
@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "6.0.0",
+    "version": "8.0.0",
     "rollForward": "latestMinor",
     "allowPrerelease": false
   }
diff --git a/src/DataStructure/Tree/Tree.cs b/src/DataStructure/Tree/Tree.cs
new file mode 100644
index 0000000..321fb0f
--- /dev/null
+++ b/src/DataStructure/Tree/Tree.cs
@@ -0,0 +1,11 @@
+namespace Skeleton.DataStructure.Tree;
+
+public class Tree<T>
+{
+    public Tree(T root)
+    {
+        Root = new TreeNode<T>(root);
+    }
+
+    public TreeNode<T> Root { get; set; }
+}
diff --git a/src/DataStructure/Tree/TreeNode.cs b/src/DataStructure/Tree/TreeNode.cs
new file mode 100644
index 0000000..eccd049
--- /dev/null
+++ b/src/DataStructure/Tree/TreeNode.cs
@@ -0,0 +1,19 @@
+namespace Skeleton.DataStructure.Tree;
+
+public class TreeNode<T>
+{
+    public static TreeNode<T> Null = null;
+    public TreeNode(T obj)
+    {
+        Value = obj;
+        Children = new();
+    }
+
+    public T Value { get; set; }
+    public HashSet<TreeNode<T>> Children { get; set; }
+
+    public void AddChild(T child)
+    {
+        Children.Add(new TreeNode<T>(child));
+    }
+}