428.序列化和反序列化 N 叉树 (Hard)*

class Codec {
public:
    // Encodes a tree to a single string.
    string serialize(Node* root) {
        if(root == nullptr) {
            return "";
        }
        string res;
        res = to_string(root->val);
        if(!root->children.empty()) {
            res += " [ ";
            auto iter = root->children.begin();
            for(; iter + 1 != root->children.end(); iter++) {
                if(*iter != nullptr) {
                    res += serialize(*iter) + " ";
                }
            }
            res += serialize(*iter);
            res += " ]"; 
        }
        return res;
    }
    // 1 [ 3 [ 5 6 ] 2 4 ]
    // Decodes your encoded data to tree.
    Node* deserialize(string data) {
        if(data.length() == 0) {
            return nullptr;
        }
        stringstream ss(data);
        stack<Node*> st;
        Node* root = nullptr;
        auto cur = root;
        string s;
        while(ss >> s) {
            if(s == "[") {
                st.push(cur);
            }else if(s == "]") {
                st.pop();
            }else {
                auto p = new Node(stoi(s));
                if(root == nullptr) {
                    root = p;
                }else {
                    auto parent = st.top();
                    parent->children.push_back(p);
                }
                cur = p;
            }
        }
        return root;
    }
};