/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {
    
    private String splitter = ",";
    private String nn = "X";

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serializeHelper(root, sb);
        return sb.toString();        
    }
    
    public void serializeHelper(TreeNode node, StringBuilder sb)
    {
        if(node == null)
            sb.append(nn).append(splitter);
        else
        {
            sb.append(node.val).append(splitter);
            serializeHelper(node.left, sb);
            serializeHelper(node.right, sb);           
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Deque<String> nodes = new LinkedList<>();
        nodes.addAll(Arrays.asList(data.split(splitter)));
        return buildTree(nodes);
    }
    
    public TreeNode buildTree(Deque<String> nodes)
    {
        String val = nodes.remove();
        if(val.equals(nn)) return null;
        else
        {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = buildTree(nodes);
            node.right = buildTree(nodes);
            return node;
        }
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));