思路

序列化：利用BFS层序遍历二叉树，节点与节点之间用 , 隔开；如果遇到空节点，则使用字母 X 代替，最终以字符串返回
反序列化：将序列化串，根据 , 分隔并保存在一个队列中；同时创建一个树节点的队列用来遍历到每层节点；遍历过程中，每次取出根节点和它可能的左右节点，然后判断这两个左右节点是否为 X ，如果是，则跳过说明为空节点；如果不是则加入其对应左右节点处，并加入队列中。

在这里插入图片描述

代码实现(java)

/**
 * 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();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while(queue.size() > 0) {
            TreeNode node = queue.poll();
            if(node == null) {
                sb.append("X").append(",");
                continue;
            }
            sb.append(node.val).append(",");
            queue.offer(node.left);
            queue.offer(node.right);
        }
        return sb.toString();
    }

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

        Queue<String> nodeQueue = new LinkedList<>(Arrays.asList(data.split(",", -1)));
        Queue<TreeNode> rootQueue = new LinkedList<>();
        TreeNode root = new TreeNode(Integer.parseInt(nodeQueue.poll()));
        rootQueue.offer(root);

        while (!rootQueue.isEmpty()) {
            TreeNode rootN = rootQueue.poll();
            String left = nodeQueue.poll();
            String right = nodeQueue.poll();

            if (!"X".equals(left)) {
                rootN.left = new TreeNode(Integer.parseInt(left));
                rootQueue.offer(rootN.left);
            }
            if (!"X".equals(right)) {
                rootN.right = new TreeNode(Integer.parseInt(right));
                rootQueue.offer(rootN.right);
            }
        }
        return root;
    }
}

// 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));