297. 二叉树的序列化与反序列化（Serialize and Deserialize Binary Tree）

/**
 * 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.
    void en_dfs(TreeNode* p, string& res) {
        if (!p) {
            res.append("n,");
            return ;
        }
        res.append(to_string(p->val)).append(",");
        en_dfs(p->left, res);
        en_dfs(p->right, res);
    }
    string serialize(TreeNode* root) {
        string res;
        en_dfs(root, res);
        return res;
    }

    // Decodes your encoded data to tree.
    void de_dfs(TreeNode*& p, int& i, vector<int>& number) {
        if (number[i] == numeric_limits<int>::max()) {
            p = nullptr;
            return ;
        }
        p = new TreeNode(number[i]);
        de_dfs(p->left, ++i, number);
        de_dfs(p->right, ++i, number);
    }
    TreeNode* deserialize(string data) {
        TreeNode* root = nullptr;
        vector<int> number;
        int i = 0;
        while (i != data.size()) {
            int j = data.find(‘,‘, i);
            string s = data.substr(i, j - i);
            if (s == "n")
                number.push_back(numeric_limits<int>::max());
            else 
                number.push_back(stoi(s));
            i = j + 1;
        }
        i = 0;
        de_dfs(root, i, number);
        return root;
    }
};

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