[leetcode]297. Serialize and Deserialize Binary Tree

题目链接：https://leetcode.com/problems/serialize-and-deserialize-binary-tree/description/

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.

Credits:
Special thanks to @Louis1992 for adding this problem and creating all test cases.

方法一：

class Codec{public:    string serialize(TreeNode* root)    {        if (!root) return " #";        string res=" "+to_string(root->val);        return res+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;    }    TreeNode* deserialize(string data)    {        istringstream is(data);        return dfs(is);    }};

方法二：

class Codec {public:    // Encodes a tree to a single string.    string serialize(TreeNode* root) {        if (root == nullptr) return "#";        return to_string(root->val)+","+serialize(root->left)+","+serialize(root->right);    }    // Decodes your encoded data to tree.    TreeNode* deserialize(string data) {        return mydeserialize(data);    }    TreeNode* mydeserialize(string& data) {        if (data[0]=='#') {            if(data.size() > 1) data = data.substr(2);            return nullptr;        } else {            TreeNode* node = new TreeNode(helper(data));            node->left = mydeserialize(data);            node->right = mydeserialize(data);            return node;        }    }private:    int helper(string& data) {        int pos = data.find(',');        int val = stoi(data.substr(0,pos));        data = data.substr(pos+1);        return val;    }};