题目：

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   5as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ 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.

分析：利用层次遍历进行序列化，在反序列化时，TreeNode数组中每个节点的left节点下标为2*(i-nullNum)+1，right节点下标为2*(i-nullNum)+2，其中nullNum为TreeNode数组中元素null的个数。

Java代码及注释如下：

// Encodes a tree to a single string.    public String serialize(TreeNode root) {      // 1,2,3,null,null,4,5,null,null,null,null        if (root == null) {            return "";        }        StringBuilder sb = new StringBuilder();        Queue
     
       queue = new LinkedList
      
       ();        queue.offer(root);        while (! queue.isEmpty()) {       //利用队列进行层次遍历(广搜)            TreeNode tmp = queue.poll();            if (tmp != null) {                sb.append(tmp.val);                queue.offer(tmp.left);                queue.offer(tmp.right);            } else {                sb.append("null");            }            sb.append(",");        }        sb.deleteCharAt(sb.length()-1);        return sb.toString();    }             // Decodes your encoded data to tree.    public TreeNode deserialize(String data) {        if ("".equals(data)) {            return null;        }        String[] tmp = data.split(",");        TreeNode[] node = new TreeNode[tmp.length]; // 根据序列化数组元素构造TreeNode数组.        for (int i = 0; i < node.length; i++) {            if ("null".equals(tmp[i])) {                node[i] = null;            } else {                node[i] = new TreeNode(Integer.parseInt(tmp[i]));            }        }        int nullNum = 0;        for (int i = 0; i < tmp.length; i++) { //找到TreeNode数组中每个node所指向的left和right节点。            if (node[i] != null) {                node[i].left = node[2*(i-nullNum)+1];                node[i].right = node[2*(i-nullNum)+2];            } else {                nullNum++;            }        }        return node[0]; //返回root    }