[leetcode] Serialize and Deserialize Binary Tree

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   5
as "[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<TreeNode> queue = new LinkedList<TreeNode>();
        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
    }