297. Serialize and Deserialize Binary Tree

问题：

用自己的方法把二叉树转化为字符串，在用自己的方法把这个字符串转为二叉树

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.

思路：

先序遍历输出二叉树，遇到空就输出null

再递归一下来吧字符串转化为树

程序：

class pointer {int pos = 0;} public class Codec {public String post(TreeNode root) {if(root == null) return "null,";else return Integer.toString(root.val)+","+post(root.left)+post(root.right);}    // Encodes a tree to a single string.    public String serialize(TreeNode root) {    return post(root);    }        public TreeNode decode(String[] str, pointer p) {    if(str[p.pos].equals("null")) {    p.pos++;    return null;    }    else {    TreeNode rt = new TreeNode(Integer.parseInt(str[p.pos]));    p.pos++;    rt.left = decode(str, p);    rt.right = decode(str, p);    return rt;    }    }    // Decodes your encoded data to tree.    public TreeNode deserialize(String data) {    String[] strs = data.split(",");    return decode(strs, new pointer());    }}