C++(BFS)

class Codec {
public:

    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        if(!root)return "";
        string res = "";
        res.append("[");
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()){
            TreeNode *node = q.front();
            q.pop();
            if(node){
                res.append(to_string(node->val));
                q.push(node->left);
                q.push(node->right);
            }
            else
                res.append("null");
            if(!q.empty())
                res.push_back(',');
        }
        res.append("]");
        return res;
    }

    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if(data.size() == 0)return NULL;
        data = data.substr(1, data.size() - 2);
        stringstream ss(data);
        vector<string> encoder;
        string str;
        while(getline(ss, str, ',')){
            encoder.push_back(str);
        }
        TreeNode *root = new TreeNode(stoi(encoder[0]));
        queue<TreeNode*> q;
        q.push(root);
        int i = 1;
        while(!q.empty()){
            TreeNode* node = q.front();
            q.pop();
            if(encoder[i] != "null"){//千万记住是"null"而不是"NULL"...
                node->left = new TreeNode(stoi(encoder[i]));
                q.push(node->left);
            }
            i ++ ;
            if(encoder[i] != "null"){
                node->right = new TreeNode(stoi(encoder[i]));
                q.push(node->right);
            }
            i ++ ;
        }
        return root;
    }
};

C++(DFS)

class Codec {
public:
    void dfs1(TreeNode* root, string& res){
        if(!root){
            res.append("null,");
            return ;
        }
        res.append(to_string(root->val));
        res.push_back(',');
        dfs1(root->left, res);
        dfs1(root->right, res);
        return ;
    }
    // Encodes a tree to a single string.
    string serialize(TreeNode* root) {
        string res;
        dfs1(root, res);
        return res;
    }

    TreeNode* dfs2(queue<string>& q){
        if(!q.empty()){
            string temp = q.front();
            q.pop();
            if(temp == "null")
                return NULL;
            TreeNode *root = new TreeNode(stoi(temp));
            TreeNode *left = dfs2(q);
            TreeNode *right = dfs2(q);
            root->left = left;
            root->right = right;
            return root;
        }
        return NULL;
    }
    // Decodes your encoded data to tree.
    TreeNode* deserialize(string data) {
        if(data.size() == 0)return NULL;
        cout << data << endl;
        stringstream ss(data);
        queue<string> encoder;
        string str;
        while(getline(ss, str, ',')){
            encoder.push(str);
        }

        return dfs2(encoder);
    }
};