[leetcode]449. Serialize and Deserialize BST

题目链接：https://leetcode.com/problems/serialize-and-deserialize-bst/

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 search tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary search tree can be serialized to a string and this string can be deserialized to the original tree structure.

The encoded string should be as compact as possible.

Note: Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

思路：使用先序遍历

class Codec {  
public:  
    // Encodes a tree to a single string.  
    string serialize(TreeNode* root) {  
        if(!root) return " #";  
        string ans = " "+to_string(root->val);  
        return ans + serialize(root->left) + serialize(root->right);   
    }  
  
    TreeNode* DFS(istringstream &is)  
    {  
        string str;  
        is >> str;  
        if(str == "#") return NULL;  
        TreeNode* root = new TreeNode(stoi(str));  
        root->left = DFS(is), root->right = DFS(is);  
        return root;  
    }  
  
    // Decodes your encoded data to tree.  
    TreeNode* deserialize(string data) {  
        istringstream is(data);  
        return DFS(is);  
    }  
};