/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        //silu,利用先序遍历，先遍历根节点，加到res中，在递归遍历左右子树
        string res;
        dfs_s(root,res);
        return res;
    }

    void dfs_s(TreeNode* root,string &res){
        if(!root){
            res += "null ";
            return;
        }

        res += to_string(root->val) + " ";
        dfs_s(root->left,res);
        dfs_s(root->right,res);
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        int u = 0;
        return dfs_d(data,u);
    }

    TreeNode* dfs_d(string data,int &u){
        if(u == data.size()) return nullptr;

        int k = u;
        while(data[k] != ' ') k++;  //k用于记录每个单词尾巴空格的位置

        //如果该单词为null,直接到下一个单词开头位置
        if(data[u] == 'n'){
            u = k + 1;
            return nullptr;
        }

        //不是null则处理数字
        int val = 0;
        if(data[u] == '-'){
         for (int i = u+1; i < k; i++) val = val * 10 + data[i] - '0';
         val  = -val;
        }
         else{
        //如果是数字是正的
            for (int i = u; i < k; i++) val = val * 10 + data[i] - '0';
        }
        u = k + 1;  //处理下一个字符串

        auto root = new TreeNode(val);
        root->left = dfs_d(data,u);
        root->right = dfs_d(data,u);
        return root;
    }
};