C ++反序列化二叉树

Question

I'm attempting to serialize and then deserialize a binary tree, but seem to have run into a roadblock during the deserialization part. I have the following code:

// Encodes a tree to a single string.
string serialize(TreeNode* root) {
    std::ostringstream oss;
    preorder_traversal(root, oss);
    return oss.str();
}

void preorder_traversal(TreeNode *root, ostringstream &oss) {
    if(root == nullptr) {
        oss << "null" << " "; // String streams are delimited by the space character
        return;
    }

    oss << root->val << " "; // String streams are delimited by the space character
    preorder_traversal(root->left, oss);
    preorder_traversal(root->right, oss);
}

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
    std::istringstream iss(data);
    TreeNode *root;
    deserialize_tree(root, iss);
    return root;
}

void deserialize_tree(TreeNode *root, istringstream &iss) {        
    string current_val;
    iss >> current_val;
    if(current_val == "null") return;

    root = new TreeNode(std::stoi(current_val)); 

    // Testing to see if root->val is being assigned the correct value
    std::cout << root->val << std::endl;

    deserialize_tree(root->left, iss);
    deserialize_tree(root->right, iss);
}; 

The problem I'm facing, is that even though std::cout << root->val << std::endl shows that values are being assigned to the newly created TreeNodes in the deserialize_tree() function, when I finally return root in the deserialize() function, I get an empty output, which has left me scratching me head quite a bit.

Note: The above code can be tested at https://leetcode.com/problems/serialize-and-deserialize-binary-tree/ if anyone would like to try and run it on some examples.

Answer 1

Maybe you need to pass the pointer and set the value it pointed to? (haven't tried it but I guess this is the problem)

// note the **root
void deserialize_tree(TreeNode **root, istringstream &iss) {        
    string current_val;
    iss >> current_val;
    if(current_val == "null") return;

    // pass pointer in and set the value it pointed to
    *root = new TreeNode(std::stoi(current_val)); 

    // Testing to see if root->val is being assigned the correct value
    std::cout << (*root)->val << std::endl;

    // pass pointer
    deserialize_tree(&root->left, iss);
    deserialize_tree(&root->right, iss);
}; 

You have to modify the prototype and everywhere call this function too

Answer 2

In code snippet which you have provided root = new TreeNode(std::stoi(current_val)); is updating local instance of root and doesn't have any impact on calling function.

Hope below snippet would work for you,

// Decodes your encoded data to tree.
TreeNode* deserialize(string data) {
    std::istringstream iss(data);
    TreeNode *root;
    deserialize_tree(root, iss);
    return root;
}

void deserialize_tree(TreeNode* &root, istringstream &iss) {        
    string current_val;
    iss >> current_val;
    if(current_val == "null") {root = nullptr;return;}

    root = new TreeNode(std::stoi(current_val)); 

     // Testing to see if root->val is being assigned the correct value
     std::cout << (root)->val << std::endl;
    deserialize_tree(root->left, iss);
    deserialize_tree(root->right, iss);
}