Merge pull request #1968 from JavieDeng/master

为0226.翻转二叉树添加了C#语言版（包含递归和迭代）

problems/0226.翻转二叉树.md

@@ -991,6 +991,53 @@ impl Solution {
 }
 ```
 
+### C#
+
+```csharp
+//递归
+public class Solution {
+    public TreeNode InvertTree(TreeNode root) {
+    if (root == null) return root;
+    
+    swap(root);
+    InvertTree(root.left);
+    InvertTree(root.right);
+    return root;
+    }
+   
+    public void swap(TreeNode node) {
+        TreeNode temp = node.left;
+        node.left = node.right;
+        node.right = temp;
+    }
+}
+```
+
+```csharp
+//迭代
+public class Solution {
+    public TreeNode InvertTree(TreeNode root) {
+          if (root == null) return null;
+        Stack<TreeNode> stack=new Stack<TreeNode>();
+        stack.Push(root);
+        while(stack.Count>0)
+        {
+            TreeNode node = stack.Pop();   
+            swap(node);
+            if(node.right!=null) stack.Push(node.right);   
+            if(node.left!=null) stack.Push(node.left);     
+        }
+        return root;
+    }
+
+    public void swap(TreeNode node) {
+        TreeNode temp = node.left;
+        node.left = node.right;
+        node.right = temp;
+    }
+}
+```
+
 <p align="center">
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>