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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary search tree can be serialized to a string and this string can be deserialized to the original tree structure.

The encoded string should be as compact as possible.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

复杂度：
时间 O(N) 空间 O(N)

思路：
采用先序遍历的方式序列化，再采用同样的先序遍历反序列化。

/**
 * 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) {
        //base case
        if(root == null)
            return "$,";
        //ensure
        return root.val + "," + serialize(root.left) + serialize(root.right);
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        //require
        Queue<String> q = new LinkedList<String>();
        //convert to string[] by " "
        String[] strs = data.split(",");
        for(String str : strs){
            q.add(str);
        }
        return dfs(q);
    }
    private TreeNode dfs(Queue<String> q){
        String str = q.remove();
        //base case
        if(str.equals("$"))
            return null;

        TreeNode root = new TreeNode(Integer.valueOf(str));
        root.left = dfs(q);
        root.right = dfs(q);
        //ensure
        return root;
    }
}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));