Update101.对称二叉树，添加C#版

problems/0101.对称二叉树.md

@@ -897,6 +897,53 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+// 递归
+public bool IsSymmetric(TreeNode root)
+{
+    if (root == null) return true;
+    return Compare(root.left, root.right);
+}
+public bool Compare(TreeNode left, TreeNode right)
+{
+    if(left == null && right != null) return false;
+    else if(left != null && right == null ) return false;
+    else if(left == null && right == null) return true;
+    else if(left.val != right.val) return false;
+
+    var outside = Compare(left.left, right.right);
+    var inside = Compare(left.right, right.left);
+
+    return outside&&inside;
+}
+```
+``` C#
+// 迭代法
+public bool IsSymmetric(TreeNode root)
+{
+    if (root == null) return true;
+    var st = new Stack<TreeNode>();
+    st.Push(root.left);
+    st.Push(root.right);
+    while (st.Count != 0)
+    {
+        var left = st.Pop();
+        var right = st.Pop();
+        if (left == null && right == null)
+            continue;
+
+        if ((left == null || right == null || (left.val != right.val)))
+            return false;
+
+        st.Push(left.left);
+        st.Push(right.right);
+        st.Push(left.right);
+        st.Push(right.left);
+    }
+    return true;
+}
+```
 
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