二叉树的层次序列化和反序列化-----stringstream

 string serialize(TreeNode* root) {//层序便利，将空的子节点也放入到字符串
 ostringstream out;
 queue<TreeNode*> q;
 q.push(root);
 while(q.size()){
 auto node = q.front();
 q.pop();
 if(!node){
 out << "null" << " ";
 continue;
 }
 out << to_string(node->val) << " ";
 q.push(node->left);
 q.push(node->right);
 }
 return out.str();
 }

 TreeNode* deserialize(string data){
 istringstream is(data);
 string val;
 vector<TreeNode*> vec;
 while(is >> val){
 if(val == "null"){
 vec.push_back(nullptr);
 continue;
 }
 auto node = new TreeNode(stoi(val));
 vec.push_back(node);
 }
 int i = ,j = ;
 while(j < vec.size()){//根据层序遍历输出的顺序得到
   if(vec[i]{
 vec[i]->left = vec[j++];//左子树
 vec[i]->right = vec[j++];//柚子树
 }
 i++;//根节点
 }
 return vec[];
 }