Serialize and Deserialize Binary Tree

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

类似于Tree的serialization最简单的方法还是依照某种遍历顺序用recursion来serialize。而因为deserialization来说preorder最方便，因为root很好找，总是第一个，所以我们优先选用preorder的遍历顺序完成serialization及deserialization。

关于deserialization, 我们可以用一个queue来存serialization的结果，每次deserialize的时候依次从queue中读取值来建node, 由于对null的node也serialize, 所以只要依照preorder的顺序deserialize, 不用担心queue中的node值与实际node不匹配。

这道题Follow up可以是N-ary Tree的serialization及deserialization, 或者类似html, xml的serialization

方法还是一样，依照某种遍历顺序用recursion来做。

复杂度

Serialization

time: O(n), space: O(1)

Deserialization

time: O(n), space: O(n)

代码

public class Codec {    // Encodes a tree to a single string.    public String serialize(TreeNode root) {        if (root == null)return "null";        StringBuilder sb = new StringBuilder();        sb.append(root.val);        String left = serialize(root.left);        String right = serialize(root.right);        sb.append(", "); // 用符号分开不同node值，方便deserialization        sb.append(left);        sb.append(", ");        sb.append(right);        return sb.toString();    }    // Decodes your encoded data to tree.    public TreeNode deserialize(String data) {    // 把所有node依照preorder serialize后的结果依次存入queue中        Queue<String> q = new LinkedList<>();        String[] strs = data.split(", ");        for (String s : strs) {q.add(s);        }        return helper(q);    }        // 从queue中依次取值建node, 顺序为preorder    public TreeNode helper(Queue<String> q) {        String s = q.remove();        if (s.equals("null"))return null;        TreeNode root = new TreeNode(Integer.parseInt(s));        root.left = helper(q);        root.right = helper(q);        return root;    }}