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.

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.

Solution:

/**
 * 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();
        pre(sb, root);
        sb.setLength(sb.length() - 1);
        return sb.toString();
    }
    
    private void pre(StringBuilder sb, TreeNode root) {
        if (root == null) {
            sb.append("#").append(",");
        } else {
            sb.append(String.valueOf(root.val)).append(",");
            pre(sb, root.left);
            pre(sb, root.right);
        }
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null || data.equals("")) return null;
        List<String> nodes = Arrays.asList(data.split(","));
        Deque<String> queue = new ArrayDeque<>(nodes);
        TreeNode root = depre(queue);
        return root;
    }
    
    private TreeNode depre(Deque<String> queue) {
        String curr = queue.poll();
        if (curr.equals("#")) {
            return null;
        } else {
            TreeNode root = new TreeNode(Integer.parseInt(curr));
            root.left = depre(queue);
            root.right = depre(queue);
            return root;
        }
    }
}

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