Codec.java

package leetcode.tree.levelorder;

import leetcode.TreeNode;

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 res = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();

                if (node == null) {
                    res.append("null").append(",");
                } else {
                    res.append(node.val).append(",");
                    queue.offer(node.left);
                    queue.offer(node.right);
                }
            }
        }

        return res.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if ("".equals(data)) {
            return null;
        }

        String[] nodes = data.split(",");
        TreeNode root = new TreeNode(Integer.parseInt(nodes[0]));
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        int index = 1;
        while (index < nodes.length) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if ("null".equals(nodes[index])) {
                    node.left = null;
                } else {
                    TreeNode left = new TreeNode(Integer.parseInt(nodes[index]));
                    node.left = left;
                    queue.offer(left);
                }
                index++;
                if ("null".equals(nodes[index])) {
                    node.right = null;
                } else {
                    TreeNode right = new TreeNode(Integer.parseInt(nodes[index]));
                    node.right = right;
                    queue.offer(right);
                }
                index++;
            }
        }

        return root;
    }
}