LeetCode 297. Serialize and Deserialize Binary Tree 二叉树的序列化与反序列化(C++/Java)

题目：

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.

Example: 

You may serialize the following tree:

    1
   / 
  2   3
     / 
    4   5

as "[1,2,3,null,null,4,5]"

Clarification: The above format is the same as how LeetCode 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：

给定一颗二叉树将其序列化，也就是转换成一个字符串，同时还需要有一个反序列化的函数用来将字符串还原成二叉树。

采用前序遍历或者层次遍历都可以，注意的是LeetCode上要求，不要使用类的成员 / 全局 / 静态变量来存储状态，你的序列化和反序列化算法应该是无状态的。

C++中可以将 string 对象作为流处理，也就是istringstream 和 ostringstream。

程序：

C++

class Codec {
public:

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        ostringstream out;
        serialize(root, out);
        return out.str();
    }
    void serialize(TreeNode* root, ostringstream& out){
        if(root != nullptr){
            out << root->val << ' ';
            serialize(root->left, out);
            serialize(root->right, out);
        }
        else{
            out << "# ";
        }
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        istringstream in(data);
        return deserialize(in);
    }
    TreeNode* deserialize(istringstream& in){
        string val;
        in >> val;
        if(val == "#"){
            return nullptr;
        }
        TreeNode* root = new TreeNode(stoi(val));
        root->left = deserialize(in);
        root->right = deserialize(in);
        return root;
    }
};

Java

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if(root == null)
            return "";
        StringBuilder str = new StringBuilder();
        serialize(root, str);
        return str.toString();
    }
    public void serialize(TreeNode root, StringBuilder str){
        if(root == null){
            str.append("#,"); 
            return;
        }
        str.append(root.val + ",");
        serialize(root.left, str);
        serialize(root.right, str);
    }
    
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if(data == "" || data == null)
            return null;
        String[] strs = data.split(",");
        LinkedList<String> l = new LinkedList<>(Arrays.asList(strs));
        return deserialize(l);
    }
    public TreeNode deserialize(LinkedList<String> list) {
        if(list.getFirst().equals("#")){
            list.removeFirst();
            return null;
        }
        TreeNode root = new TreeNode(Integer.parseInt(list.getFirst()));
        list.removeFirst();
        root.left = deserialize(list);
        root.right = deserialize(list);
        return root; 
    }
    
}