/**
 * 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) {
        //序列化与反序列化
        //如果为空节点,往res 中加null即可，如果非空，加上本身的值，对左右子树进行递归
        //反序列化，如果为null则直接返回空节点，处理下一个即可，如果u == size 说明处理完毕
        //返回即可，如果非空则对左右继续处理
        string res;
        dfs_s(root,res);
        //cout << res << endl;
        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++;

        if(data[u] == 'n'){
            u = k + 1;
            return nullptr;
        }

        int val = 0;
        if(data[u] == '-'){
            for(int i = u + 1;i < k;++i)    val = 10 * val + data[i] - '0';
            val *= -1; 
        }
        else{
            for(int i = u;i < k;++i)    val = 10 * val + data[i] - '0';
        }
        u = k + 1;
        TreeNode* root = new TreeNode(val);
        root->left = dfs_d(data,u);
        root->right = dfs_d(data,u);
        return root;
    }
};