Day i do not know

101. Symmetric Tree/101. Symmetric Tree.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <RootNamespace>_101._Symmetric_Tree</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

101. Symmetric Tree/Program.cs

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+namespace _101._Symmetric_Tree;
+
+internal class Program
+{
+    static void Main(string[] args)
+    {
+        var t = new TreeNode(3, 
+            new TreeNode(4, new TreeNode(5, new TreeNode(6), null), null), 
+            new TreeNode(4, null, new TreeNode(5, null, new TreeNode(6)))
+            );
+        Console.WriteLine(new Solution().IsSymmetric(t));
+    }
+}

101. Symmetric Tree/Solution.cs

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+namespace _101._Symmetric_Tree;
+
+public class Solution
+{
+    public bool IsSymmetric(TreeNode root)
+    {
+        return Recursion(root.left, root.right);
+    }
+
+    private bool Recursion(TreeNode? left, TreeNode? right)
+    {
+        if ((left == null) != (right == null))
+            return false;
+        if (left == null || right == null)
+            return true;
+        if (left.val != right.val)
+            return false;
+        return Recursion(left.left, right.right) && Recursion(left.right, right.left);
+    }
+}

101. Symmetric Tree/TreeNode.cs

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+namespace _101._Symmetric_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;
+    }
+}
+