Day i do not know

226. Invert Binary Tree/226. Invert Binary Tree.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <RootNamespace>_226._Invert_Binary_Tree</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

226. Invert Binary Tree/Program.cs

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+namespace _226._Invert_Binary_Tree;
+
+internal class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine("Hello, World!");
+    }
+}

226. Invert Binary Tree/Solution.cs

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+namespace _226._Invert_Binary_Tree;
+
+public class Solution
+{
+    public TreeNode? InvertTree(TreeNode root)
+    {
+        Stack<TreeNode> stack = new Stack<TreeNode>();
+        if (root != null)
+            stack.Push(root);
+        while (stack.Count > 0)
+        {
+            TreeNode cur = stack.Pop();
+            TreeNode? buf = cur.left;
+            cur.left = cur.right;
+            cur.right = buf;
+            if (cur.right != null)
+                stack.Push(cur.right);
+            if (cur.left != null)
+                stack.Push(cur.left);
+        }
+        return root;
+    }
+}

226. Invert Binary Tree/TreeNode.cs

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+namespace _226._Invert_Binary_Tree;
+
+
+//Definition for a binary tree node.
+public class TreeNode
+{
+    public int val;
+    public TreeNode? left;
+    public TreeNode? right;
+    public TreeNode(int val = 0, TreeNode? left = null, TreeNode? right = null)
+    {
+        this.val = val;
+        this.left = left;
+        this.right = right;
+    }
+}