# Binary Tree Serialization ### Source - lintcode: [(7) Binary Tree Serialization](http://www.lintcode.com/en/problem/binary-tree-serialization/) ~~~ Design an algorithm and write code to serialize and deserialize a binary tree. Writing the tree to a file is called 'serialization' and reading back from the file to reconstruct the exact same binary tree is 'deserialization'. There is no limit of how you deserialize or serialize a binary tree, you only need to make sure you can serialize a binary tree to a string and deserialize this string to the original structure. Have you met this question in a real interview? Yes Example An example of testdata: Binary tree {3,9,20,#,#,15,7}, denote the following structure: 3 / \ 9 20 / \ 15 7 Our data serialization use bfs traversal. This is just for when you got wrong answer and want to debug the input. You can use other method to do serializaiton and deserialization. ~~~ ### 題解 根據之前由前序，中序，后序遍歷恢復二叉樹的經驗，確定根節點的位置十分重要（但是這里可能有重復元素，故和之前的題目不太一樣）。能直接確定根節點的有前序遍歷和廣度優先搜索，其中較為簡潔的為前序遍歷。序列化較為簡單，但是反序列化的實現不太容易。需要借助字符串解析工具。 ### Java ~~~ /** * Definition of TreeNode: * public class TreeNode { * public int val; * public TreeNode left, right; * public TreeNode(int val) { * this.val = val; * this.left = this.right = null; * } * } */ class Solution { /** * This method will be invoked first, you should design your own algorithm * to serialize a binary tree which denote by a root node to a string which * can be easily deserialized by your own "deserialize" method later. */ public String serialize(TreeNode root) { StringBuilder sb = new StringBuilder(); if (root == null) return sb.toString(); seriaHelper(root, sb); return sb.substring(0, sb.length() - 1); } private void seriaHelper(TreeNode root, StringBuilder sb) { if (root == null) { sb.append("#,"); } else { sb.append(root.val).append(","); seriaHelper(root.left, sb); seriaHelper(root.right, sb); } } /** * This method will be invoked second, the argument data is what exactly * you serialized at method "serialize", that means the data is not given by * system, it's given by your own serialize method. So the format of data is * designed by yourself, and deserialize it here as you serialize it in * "serialize" method. */ public TreeNode deserialize(String data) { if (data == null || data.length() == 0) return null; StringTokenizer st = new StringTokenizer(data, ","); return deseriaHelper(st); } private TreeNode deseriaHelper(StringTokenizer st) { if (!st.hasMoreTokens()) return null; String val = st.nextToken(); if (val.equals("#")) { return null; } TreeNode root = new TreeNode(Integer.parseInt(val)); root.left = deseriaHelper(st); root.right = deseriaHelper(st); return root; } } ~~~ ### 源碼分析 由二叉樹序列化的過程不難，難就難在根據字符串進行反序列化，這里引入了 Java 中的 StringTokenizer 字符串分割工具，非常方便，使得遞歸得以順利實現。其中`deseriaHelper`的實現較為巧妙。 ### 復雜度分析 略 ### Reference - [Serialize and Deserialize a Binary Tree (pre order).](https://gist.github.com/bittib/5620951) - [Serialization/Deserialization of a Binary Tree | LeetCode](http://articles.leetcode.com/2010/09/serializationdeserialization-of-binary.html)