CODE

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */

public class Codec {

  /* format: root - left - right:
    ro le ri
	ro(Val) le (ro(Val)) ri (ro(Val))
	N = null
	recursive
	e.g., ro1lero2leNriNriro3leNriN 
	  means root 1 has children 2 and 3
	  which are leaf nodes */

  // Encodes a tree to a single string.
  public string Serialize(TreeNode root) {

	if (root == null) return "N";

	string ro = "ro" + root.val.ToString() + "le";
	string le = Serialize(root.left) + "ri";
    string ri = Serialize(root.right);

	return ro + le + ri;        
  }

  // Decodes your encoded data to tree.
  public TreeNode Deserialize(string data) {
	if (data.Equals("N")) return null;

	int rootVal = data[2] - '0';
	TreeNode root = new TreeNode(rootVal);

	List<TreeNode> nodes = new List<TreeNode>();
	nodes.Add(root);

	int i = 4;
	/* go through serialized string */
	while (i < data.Length)
	{
	  /* process left child */
	  if (data[i] == 'e')
	  {
	    /* not null, get value of left child */
		if (data[i+1] == 'r')
		{
		  int k = i+3;
		  /* val may be more than 1 digit */
		  while (Char.IsNumber(data[k])) k++;
		  string valSubStr = data.Substring(i+3,k-(i+3));
		  int val = Int32.Parse(valSubStr);

		  /* create new left child and assign to parent */
		  TreeNode n = new TreeNode(val);
		  nodes[nodes.Count-1].left = n;
		  nodes.Add(n);
		  
		  /* advance to right child */
		  i += (4 + valSubStr.Length);
		}
		else i += 3; /* left child is null, go to right child */
	  }
	  /* process right child */
	  else if (data[i] == 'i')
	  {
		/* right child not null */
		if (data[i+1] == 'r')
		{
		  int k = i+3;
		  while (Char.IsNumber(data[k])) k++;
		  string valSubStr = data.Substring(i+3,k-(i+3));
		  int val = Int32.Parse(valSubStr);
		  TreeNode n = new TreeNode(val);
		  nodes[nodes.Count-1].right = n;
		  
		  /* parent node now has both children set, 
			 pop it for backtrack */                     
		  nodes.RemoveAt(nodes.Count-1);
		  nodes.Add(n); /* add right child to nodes */
		  i += (4 + valSubStr.Length);
		}
		else 
		{
		  /* right child is null, pop parent for backtrack */
		  nodes.RemoveAt(nodes.Count-1);                
		  i += 3;
		}
	  }
	}        
    return root;
  }
}