二叉树的序列化与反序列化

顺便说一下，今天做题的时候也用到了stringstream这个类，是二叉树的序列化和反序列化。

　　题目链接：http://www.lintcode.com/zh-cn/problem/binary-tree-serialization/

v二叉树的序列化和反序列化

　　设计一个算法，并编写代码来序列化和反序列化二叉树。将树写入一个文件被称为“序列化”，读取文件后重建同样的二叉树被称为“反序列化”。如何反序列化或序列化二叉树是没有限制的，你只需要确保可以将二叉树序列化为一个字符串，并且可以将字符串反序列化为原来的树结构。

v思路：

　　通过先序遍历建立二叉树的序列化，其中空子树用'#'来表示。反序列化的时候呢，遇到'#'就停止递归构造。另外序列化的时候是将整数通过stringstream转换成字符串，反序列化是将字符串通过stringstream转换成整数。

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
public:
    /**
     * 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.
     */
    bool first;
    
    template<typename out_type, typename in_value>
    out_type convert(const in_value & t){
        stringstream stream;
        stream<<t;//向流中传值
        out_type result;//这里存储转换结果
        stream>>result;//向result中写入值
        return result;
    }
    
    void pre_order(TreeNode *root, string &s){
        if(root){
            string tmp = convert<string>(root->val);
            if(!first)
                s+= " "+tmp;
            else {
                first = false;
                s+=tmp;
            }
            pre_order(root->left, s);
            pre_order(root->right, s);
        } else {
            if(first)
                s+='#';
            else {
                first = false;
                s+=" #";
            }
        }
    }
    string serialize(TreeNode *root) {
        // write your code here
        string s="";
        first = true;
        pre_order(root, s);//先序实现序列化
        return s;
    }
    
    stringstream ss;
    void buildT(TreeNode * &T){
        string s;
        ss>>s;
        if(s == "#") return ;
        int val = convert<int>(s);
        T = new TreeNode(val);
        buildT(T->left);
        buildT(T->right);
    }
    
    /**
     * 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.
     */
    TreeNode *deserialize(string data) {
        // write your code here
        TreeNode *T = NULL;
        ss.str("");
        ss<<data;
        buildT(T);
        return T;
    }
};