string流

istringstream和ostringstream

　　从istringstream类中读取数据赋值给某个string，写入某个string到ostringstream类，头文件<sstream>

实例：leetcode297

297. Serialize and Deserialize Binary Tree 序列化和反序列化二叉树
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.
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.
将一个二叉树的值转化为一个string输出，同时又能把这种string转化二叉树；
前序、中序、后序、层序遍历均可；

法一：
非递归层序遍历以及由层序遍历来构造二叉树；
主要要使用string流，不用string流的话，对于像"[11,-2,3,null,null,42,5]"这样val值大于9或者val值为负数的，你不好处理；

/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(TreeNode* root)
    {
        if (root == NULL)
            return "#";
        ostringstream out;
        queue<TreeNode*> que;
        que.push(root);
        while (!que.empty())
        {
            TreeNode* cur = que.front();
            que.pop();
            if (cur)
            {
                out << to_string(cur->val) << " ";//向ostringstream类out中写入，记住要写空格字符串“ ”
                que.push(cur->left);
                que.push(cur->right);
            }
            else
                out << "# ";//记住要写空格字符串“ ”
        }
        return out.str();
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data)
    {
        if (data == "#")
            return NULL;
        istringstream in(data);
        queue<TreeNode*> que;
        string valString;
        in >> valString;//从istringstream类in中读取一个string赋值给valString，istringstream类默认以空格为分隔符
        TreeNode* root = new TreeNode(stoi(valString));
        que.push(root);
        while (!que.empty())
        {
            TreeNode* cur = que.front();
            que.pop();
            in >> valString;
            if (valString == "")
                break;
            if (valString != "#")
            {
                cur->left = new TreeNode(stoi(valString));
                que.push(cur->left);
            }
            in >> valString;
            if (valString == "")
                break;
            if (valString != "#")
            {
                cur->right = new TreeNode(atoi(valString.c_str()));
                que.push(cur->right);
            }
        }
        return root;
    }
};

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