简化版前序遍历,序列化过程中将空结点记为’n’,每个结点用空格隔开

class Solution {
public:

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        string res = dfs_s(root);
        return res;
    }

    string dfs_s(TreeNode* root)
    {
        if(!root) return "n ";
        return to_string(root -> val) + ' ' + dfs_s(root -> left) + dfs_s(root -> right);
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        auto res = dfs_d(data);
        return res;
    }

    TreeNode* dfs_d(string& s)
    {
        if(s[0] == 'n')     //若以n开头，返回空结点，删掉开头的n和一个空格
        {
            s = s.substr(2);
            return NULL;
        }
        int val = 0, k = 0, fu = 0;
        if(s[0] == '-') k++, fu = 1;    // 如果是负数则，k后移一位，并记下符号位

        while(s[k] != ' ') val = val * 10 + s[k++] - '0' ; 
        if(fu) val *= -1;
        s = s.substr(k + 1);    // 将该数删掉

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

};