注意数字是负数的情况，y神视频中似乎没提到
```
class Solution {
public:

// Encodes a tree to a single string.
string serialize(TreeNode* root) { //用前序序列化
    string res;
    dfs_s(root, res);
    return res;
}
void dfs_s(TreeNode* root, string& s){
    if(!root){
        s += "null ";
        return;
    }
    s += to_string(root->val) + ' ';
    dfs_s(root->left, s);
    dfs_s(root->right, s);
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) { //反序列化
    int u = 0;
    return dfs_d(data, u);
}
TreeNode* dfs_d(string data, int &u){
    if(u == data.size()) return NULL;
    int k = u;
    while(data[k] != ' ') k++;
    if(data[u] == 'n'){
        u = k + 1;
        return NULL;
    }

    int val = 0, flag = 0;
    for(int i = u; i < k; i++){
         if(data[i] == '-') { //做个标记
             flag = 1;
             continue;
         }
         val = val * 10 + data[i] - '0';
    }
    u = k + 1;
    if(flag) val = -val; //标记使用
    auto root = new TreeNode(val);
    root->left = dfs_d(data, u);
    root->right = dfs_d(data, u);
    return root;
}

};
```