public class Codec

     {

         // Encodes a tree to a single string.

         public string serialize(TreeNode root)

         {

             if (root == null)

                 return "";

             Queue<TreeNode> queue = new Queue<TreeNode>();

             StringBuilder result = new StringBuilder();

             queue.Enqueue(root);

             while (true)

             {

                 if (queue.Count() == )

                     break;

                 int nodesAtLevel = queue.Count();

                 StringBuilder list = new StringBuilder();

                 for (int i = ; i < nodesAtLevel; i++)

                 {

                     var node = queue.Dequeue();

                     if (node != null)

                     {

                         list.Append(node.val.ToString() + ",");

                         if (node.left != null)

                             queue.Enqueue(node.left);

                         else

                             queue.Enqueue(null);

                         if (node.right != null)

                             queue.Enqueue(node.right);

                         else

                             queue.Enqueue(null);

                     }

                     else

                     {

                         list.Append("null" + ",");

                     }

                 }

                 result.Append(list.ToString());

             }

             result.Remove(result.Length - , );

             return result.ToString();

         }

         // Decodes your encoded data to tree.

         public TreeNode deserialize(string data)

         {

             if (data.Length == )

                 return null;

             Queue<TreeNode> queue = new Queue<TreeNode>();

             var tlist = data.Split(',');

             var count = tlist.Length;

             foreach (var l in tlist)

             {

                 if (l != "null")

                 {

                     var t = new TreeNode(int.Parse(l));

                     queue.Enqueue(t);

                 }

                 else

                 {

                     queue.Enqueue(null);

                 }

             }

             var root = queue.Dequeue();

             var list = new List<TreeNode>();

             list.Add(root);

             while (queue.Any())

             {

                 var temp = new List<TreeNode>();

                 for (int i = ; i < list.Count; i++)

                 {

                     var p = list[i];

                     var left = queue.Dequeue();

                     if (left != null)

                     {

                         p.left = left;

                     }

                     var right = queue.Dequeue();

                     if (right != null)

                     {

                         p.right = right;

                     }

                     if (p.left != null)

                     {

                         temp.Add(p.left);

                     }

                     if (p.right != null)

                     {

                         temp.Add(p.right);

                     }

                 }

                 list = temp;

             }

             return root;

         }

     }