class Codec {
public:

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if (root == NULL) return "#";
        return to_string(root->val) + "," + serialize(root->left) + "," + serialize(root->right);
    }

    TreeNode* deserialize(string data) {
        if (data == "#") return NULL;
        stringstream s(data);
        return makeDeserialize(s);
    }

    TreeNode* makeDeserialize(stringstream& s) {
        string str;
        getline(s, str, ',');
        if (str == "#") {
            return NULL;
        } else {
            TreeNode* root = new TreeNode(stoi(str));
            root->left = makeDeserialize(s);
            root->right = makeDeserialize(s);
            return root;
        }
    }
};

REF:

• https://discuss.leetcode.com/topic/32470/clean-c-solution/2