LeetCode: Serialize Deserialize Binary Tree

/**
Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

Example:

You may serialize the following tree:

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]"
Clarification: The above format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.
*/
import java.util.*;

class TreeNode{
TreeNode left, right;
int val;
public TreeNode(int v){
val = v;
left = null;
right = null;
}

}
public class SerializeDeserializeBinaryTree{

// Encodes a tree to a single string.
    public static String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serializeHelper(root, sb);
        return sb.toString();
    }
 
    static void serializeHelper(TreeNode root, StringBuilder sb){
        if(root==null){
//append NN for null node
            sb.append("NN").append(" ");
            return;
        }
        sb.append(root.val).append(" ");
        serializeHelper(root.left, sb);
        serializeHelper(root.right, sb);
    }
    // Decodes your encoded data to tree.
//add all the encoded nodes into a dequeue and recursively build the tree
    public static TreeNode deserialize(String data) {
        Deque<String> nodes = new LinkedList<>();
        nodes.addAll(Arrays.asList(data.split(" ")));
        return buildTree(nodes);
    }
 
//
    private static TreeNode buildTree(Deque<String> nodes) {
        String val = nodes.remove();
// NN was used for null nodes during the encoding
        if (val.equals("NN")) return null;
        else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = buildTree(nodes);
            node.right = buildTree(nodes);
            return node;
        }
    }

public static void main(String args[]){
TreeNode root = new TreeNode(1);
root.left = new TreeNode(2);
root.right = new TreeNode(3);
root.right.left = new TreeNode(4);
root.right.right = new TreeNode(5);
String ser = serialize(root);
System.out.println(ser);
TreeNode n = deserialize(ser);
Queue<TreeNode> q = new LinkedList<TreeNode>();
q.add(n);
while(!q.isEmpty()){
TreeNode cur = q.poll();
if(cur!=null){
System.out.println(cur.val);
if(cur.left!=null);
q.add(cur.left);
if(cur.right!=null)
q.add(cur.right);
}

}
}
}