LeetCode 449 Serialize and Deserialize BST

use level order like we always do for problems like ser and de-ser.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return "";
        
        StringBuilder sb = new StringBuilder();
        
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        
        while (!q.isEmpty()) {
            TreeNode node = q.poll();
            if (node == null) {
                sb.append("n ");
                continue;
            }
            sb.append(node.val + " ");
            q.add(node.left);
            q.add(node.right);
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.length() == 0) {
            return null;
        }
        
        String[] s = data.split(" ");
        
        TreeNode root = new TreeNode(Integer.parseInt(s[0]));
        
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        
        for (int i = 1; i < s.length; i++) {
            TreeNode parent = q.poll();
            if (!s[i].equals("n")) {
                parent.left = new TreeNode(Integer.parseInt(s[i]));
                q.add(parent.left);
            }
            i++;
            if (i < s.length && !s[i].equals("n")) {
                parent.right = new TreeNode(Integer.parseInt(s[i]));
                q.add(parent.right);
            }
        }
        return root;
    }
}