Serialize and 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.

For example, you may serialize the following tree

    1
   / \
  2   3
     / \
    4   5

as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ 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.

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        return sb.toString();
    }
    
    private void buildString(TreeNode root, StringBuilder sb) {
        if (root == null) {
            sb.append("N" + ",");
            return;
        }
        sb.append(root.val + ",");
        buildString(root.left, sb);
        buildString(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        Queue<String> nodes = new LinkedList<>();
        nodes.addAll(Arrays.asList(data.split(",")));
        return buildTree(nodes);
    }
    
    private TreeNode buildTree(Queue<String> queue) {
        String val = queue.poll();
        if (val.equals("N")) {
            return null;
        } else {
            TreeNode node = new TreeNode(Integer.valueOf(val));
            node.left = buildTree(queue);
            node.right = buildTree(queue);
            return node;
        }
        
    }
}