leetcode297.Serialize and Deserialize Binary Tree

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题目要求

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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.

设计一个方法将一个二叉树序列化并反序列化。序列化是指将对象转化成一个字符串，该字符串可以在网络上传输，并且到达目的地后可以通过反序列化恢复成原来的对象。

在Leetcode中，树的序列化是通过广度优先遍历实现的，就如题目中所介绍的那样。

这里我们使用中序遍历来解决这个问题。

##思路和代码##
中序遍历是指递归的先遍历当前节点，然后按同样的方式递归的遍历左子树，再递归的遍历右子树。因此题目中的例子的中序遍历结果为[1,2,3,4,5]。但是，标准的中序遍历并不能使我们将该结果反构造成一棵树，我们丢失了父节点和子节点之间的关系。因此我们需要适当的插入空值来保存父节点和子节点的关系。

因此，我们将访问到空值也记录下来，结果如下[1,2, , ,3,4, , ,5, , ]。

这里我们也可以明显的看出来，中序遍历需要保存的空值远远多于广度优先遍历。

那么我们如何将这个字符串反序列化呢？
其实思路还是一样的，将其根据分隔符分割后传入队列，不断从队列头读取数据，先构建当前节点，然后依次构建左子树和右子树，如果遇到空值，则返回递归。

代码如下：

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private static final String NULL = "N";
private static final String SPLITOR = ",";

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

private void inOrder(TreeNode root, StringBuilder result){
    if(root==null){
        result.append(NULL).append(SPLITOR);
    }else{
        result.append(root.val).append(SPLITOR);
        inOrder(root.left, result);
        inOrder(root.right, result);
    }
}

// Decodes your encoded data to tree.
public TreeNode deserialize(String data) {
    Deque<String> nodes = new LinkedList<String>(Arrays.asList(data.split(SPLITOR)));
    return buildTree(nodes);
}

private TreeNode buildTree(Deque<String> nodes) {
    String val = nodes.remove();
    if (val.equals(NULL)) return null;
    else {
        TreeNode node = new TreeNode(Integer.valueOf(val));
        node.left = buildTree(nodes);
        node.right = buildTree(nodes);
        return node;
    }
}