Serialize and deserialize binary tree -- Leetcode

Question:

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

Answer:
Method: Preorder and DFS!
public class Codec {
    private StringBuilder sb = new StringBuilder();
    private int index = 0;
   
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        index = 0;
        serialize(root, sb);
        return sb.toString();
    }

    public void serialize(TreeNode root, StringBuilder sb){
        //preorder and using '# ' for null node!
        if(root==null){
            sb.append("#,");
            return;
        }
       
        sb.append(root.val + ",");
       
        serialize(root.left, sb);
        serialize(root.right, sb);
        return;
    }
   
   
   
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data==null || data.length()==0){
            return null;
        }
        String[] strArr = data.split(",");
        return deserialize(strArr);
    }
   
   
    public TreeNode deserialize(String[] str){
        if(index >= str.length){
            return null;
        }
       
        String str_val = str[index];
       
        if(str_val.equals("#")){
            index++;
            return null;
        }

        TreeNode root = new TreeNode(Integer.parseInt(str_val));
        index++;
        root.left = deserialize(str);
        root.right = deserialize(str);
        return root;
    }
}

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