Update 110.平衡二叉树，添加C#版

problems/0110.平衡二叉树.md

@@ -908,6 +908,31 @@ impl Solution {
     }
 }
 ```
+### C#
+```C#
+public bool IsBalanced(TreeNode root)
+{
+    return GetHeight(root) == -1 ? false : true;
+}
+public int GetHeight(TreeNode root)
+{
+    if (root == null) return 0;
+    int left = GetHeight(root.left);
+    if (left == -1) return -1;
+    int right = GetHeight(root.right);
+    if (right == -1) return -1;
+    int res;
+    if (Math.Abs(left - right) > 1)
+    {
+        res = -1;
+    }
+    else
+    {
+        res = 1 + Math.Max(left, right);
+    }
+    return res;
+}
+```
 
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