Codec.java

import java.util.Arrays;
import java.util.LinkedList;
import java.util.Queue;

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return "";
        StringBuilder sb = new StringBuilder();
        serialize(root, sb);
        return sb.toString();
    }

    private void serialize(TreeNode root, StringBuilder sb){
        if (root == null) return;
        sb.append(root.val).append(",");
        serialize(root.left, sb);
        serialize(root.right, sb);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.isEmpty()) return null;
        Queue<String> queue = new LinkedList<>(Arrays.asList(data.split(",")));
        return deserialize(queue, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    private TreeNode deserialize(Queue<String> queue, int min, int max){
        if (queue.isEmpty()) return null;
        int val = Integer.parseInt(queue.peek());
        if (val < min || val > max) return null;
        queue.poll();
        TreeNode root = new TreeNode(val);
        root.left = deserialize(queue, min, val);
        root.right = deserialize(queue, val, max);
        return root;
    }
}