完善所以c#相关的文档和代码

codes/csharp/include/TreeNode.cs

@@ -0,0 +1,92 @@
+namespace hello_algo.include
+{
+    public class TreeNode
+    {
+        public int? val;        // 结点值
+        public int height;     // 结点高度
+        public TreeNode? left;  // 左子结点引用
+        public TreeNode? right; // 右子结点引用
+
+        public TreeNode(int? x)
+        {
+            val = x;
+        }
+
+        /**
+         * Generate a binary tree with an array
+         * @param arr
+         * @return
+         */
+        public static TreeNode? arrToTree(int?[] arr)
+        {
+            if (arr.Length == 0)
+                return null;
+
+            TreeNode root = new TreeNode(arr[0]);
+            Queue<TreeNode> queue = new Queue<TreeNode>();
+            queue.Enqueue(root);
+            int i = 1;
+            while (queue.Count!=0)
+            {
+                TreeNode node = queue.Dequeue();
+                if (arr[i] != null)
+                {
+                    node.left = new TreeNode(arr[i]);
+                    queue.Enqueue(node.left);
+                }
+                i++;
+                if (arr[i] != null)
+                {
+                    node.right = new TreeNode(arr[i]);
+                    queue.Enqueue(node.right);
+                }
+                i++;
+            }
+            return root;
+        }
+
+        /**
+         * Serialize a binary tree to a list
+         * @param root
+         * @return
+         */
+        public static List<int?> treeToList(TreeNode root)
+        {
+            List<int?> list = new();
+            if (root == null) return list;
+            Queue<TreeNode?> queue = new();
+            while (queue.Count != 0)
+            {
+                TreeNode? node = queue.Dequeue();
+                if (node != null)
+                {
+                    list.Add(node.val);
+                    queue.Enqueue(node.left);
+                    queue.Enqueue(node.right);
+                }
+                else
+                {
+                    list.Add(null);
+                }
+            }
+            return list;
+        }
+
+        /**
+         * Get a tree node with specific value in a binary tree
+         * @param root
+         * @param val
+         * @return
+         */
+        public static TreeNode? getTreeNode(TreeNode? root, int val)
+        {
+            if (root == null)
+                return null;
+            if (root.val == val)
+                return root;
+            TreeNode? left = getTreeNode(root.left, val);
+            TreeNode? right = getTreeNode(root.right, val);
+            return left != null ? left : right;
+        }
+    }
+}