必须返回char*的接口

/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};
*/
class Solution {
public:
    char* Serialize(TreeNode *root) {    
        if (root == nullptr) return nullptr;
        string str;
        queue<TreeNode*> q; // bfs
        q.push(root);
        while (!q.empty()) {
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                auto node = q.front();
                q.pop();
                if (node != nullptr) {
                    str += to_string(node->val) + ","; // 加,是为了区分开节点值
                    q.push(node->left); // 左子树是null也入队
                    q.push(node->right);
                } else {
                    str += "#";
                }
            }
        }
        auto res = new char[str.length() + 1];
        strcpy(res, str.c_str());
        res[str.length()] = '\0';      
        return res; 
    }
    TreeNode* Deserialize(char *str) {
        if (str == nullptr) return nullptr;
        int k = 0;
        queue<TreeNode*> q;
        auto root = new TreeNode(getvalue(str, k));
        q.push(root);
        while (!q.empty()) {
            int size = q.size();
            for (int i = 0; i < size; ++i) {
                auto node = q.front();
                q.pop();
                int l = getvalue(str, k);
                if (l != -1) {
                    auto node_l = new TreeNode(l);
                    node->left = node_l;
                    q.push(node_l); 
                }
                int r = getvalue(str, k);
                if (r != -1) {
                    auto node_r = new TreeNode(r);
                    node->right = node_r;
                    q.push(node_r); 
                }
            }
        }
        return root;
    }

    int getvalue(char* str, int &k) {
        int val = 0;
        while (str[k] != ',' && str[k] != '\0' && str[k] != '#') {
            val = val * 10 + str[k] - '0';
            k++;
        }
        if (str[k] == '\0') return -1;
        if (str[k] == '#') val = -1;
        k++;
        return val;
    }


};

leetcode

class Codec {
public:
    // [1,2,3,null,null,4,5]
    /*
        res == 2,#,#
        res == 4,#,#
        res == 5,#,#
        res == 3,4,#,#,5,#,#
        res == 1,2,#,#,3,4,#,#,5,#,#
    */
    string serialize(TreeNode* root) {
        if (root == nullptr) return "#";
        string res = to_string(root->val);
        res += "," + serialize(root->left);
        res += "," + serialize(root->right);
        cout << "res == " << res << endl;
        return res;
    }
    string m_data;
    int m_index = 0;
    TreeNode* dfs_rebuild() {
        if (m_data[m_index] == '#') {
            m_index += 2;
            return nullptr;
        }
        TreeNode* root = new TreeNode(-1);
        int sum = 0;
        int j = m_index;
        // [0,9]保证了一个节点的值不会被序列化时候的,造成影响
        while (j < m_data.size() && m_data[j] >= '0' && m_data[j] <= '9') {
            sum = sum * 10 + (m_data[j] - '0');
            j++;
        }
        m_index = j+1;
        root->val = sum;
        root->left = dfs_rebuild();
        root->right = dfs_rebuild();
        return root;
    }
   
    TreeNode* deserialize(string data) {
        m_data = data;
        return dfs_rebuild();
    }
};