297. Serialize and Deserialize Binary Tree!

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        serializeHelp(root, sb);
        return sb.toString();
    }
    
    private void serializeHelp(TreeNode node, StringBuilder sb){
        // use preorder dfs, 
        // so that the root node is created before the childern during deserialization
        // "N" for null, " " for split
        if(node == null){
            sb.append("N ");
            return;
        }
        sb.append(node.val+" ");
        serializeHelp(node.left, sb);
        serializeHelp(node.right, sb);
    } 

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        List<String> strList = Arrays.asList(data.split(" "));
        Queue<String> list = new LinkedList<>(strList);
        return deserializeHelp(list);
    }
    
    private TreeNode deserializeHelp(Queue<String> list){
        // dfs and bulid the tree same order as serialization
        String str = list.poll();
        if(str.equals("N")) return null;
        
        TreeNode node = new TreeNode(Integer.parseInt(str));
        node.left=deserializeHelp(list);
        node.right=deserializeHelp(list);
        return node;
    }
}

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