Serialize and deserialize Binary Tree

Dec 18, 2024 • 5 mins read

Problem Overview

LeetCode Problem: Serialize and Deserialize Binary Tree

In this problem, you are required to convert a binary tree to a string representation (serialization) and then back to a binary tree (deserialization). This is a common problem in computer science for data persistence and network transmission, allowing efficient storage and communication of tree structures.

Understanding the Problem

Serialization is the process of converting a data structure or object into a format that can be easily stored or transmitted, and later reconstructed. Deserialization is the reverse process: transforming a serialized form back into its original data structure.

For a binary tree, the challenge lies in representing the hierarchical nature of the tree in a linear form, with enough information that the original tree can be recreated exactly. The tree can contain any number of nodes and has a recursive structure, making it non-trivial to serialize and deserialize.

Key DSA Concepts and Theory

Binary Tree: A data structure where each node has at most two children, referred to as the left and right child.
Tree Traversal: Methods to visit all nodes in a tree. Preorder, inorder, and postorder are common techniques. Preorder traversal is particularly useful here because it preserves the root-to-leaf path during serialization.
Queue: A data structure following the first-in-first-out (FIFO) principle. Used in this problem to assist with level-order traversal during the serialization and deserialization processes.
Recursion and Iteration: Recursive methods can navigate through tree structures easily, while iterative methods with explicit stacks or queues avoid deeper recursion depths.

Solution Approach

To solve this problem, you need two primary methods: serialize, which converts the tree to a string, and deserialize, which builds the tree from the string.

Serialize (Tree to String)

Choose Traversal Method: Level-order traversal is efficient for serialization as it represents nodes level by level, providing a natural order similar to how trees grow.
Use a Queue: Utilize a queue to facilitate level-order traversal. Start from the root and iterate over the tree.
Handle Nulls: Use a designated symbol (e.g., "null") to denote absent children (in cases where the tree isn't complete).
Build the String: Append values sequentially, separated by delimiters, to construct the desired string format.

C++ Code for Serialization

string serialize(TreeNode* root) {
    if (!root) return "";
    queue<TreeNode*> q;
    q.push(root);
    string result = "";
    while (!q.empty()) {
        TreeNode* node = q.front();
        q.pop();
        if (node) {
            result += to_string(node->val) + ",";
            q.push(node->left);
            q.push(node->right);
        } else {
            result += "null,";
        }
    }
    result.pop_back(); // remove last comma
    return result;
}

Java Code for Serialization

public String serialize(TreeNode root) {
    if (root == null) return "";
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    StringBuilder sb = new StringBuilder();
    while (!queue.isEmpty()) {
        TreeNode node = queue.poll();
        if (node != null) {
            sb.append(node.val).append(",");
            queue.offer(node.left);
            queue.offer(node.right);
        } else {
            sb.append("null,");
        }
    }
    sb.setLength(sb.length() - 1); // remove last comma
    return sb.toString();
}

Deserialize (String to Tree)

Split the String: Use the delimiter to split the string into parts.
Initialize the Queue: Start with the first value as the root node, and initiate a queue to assist with level-order reconstruction.
Rebuild the Tree: Iteratively attach child nodes by reading subsequent values, using "null" to skip non-existent children.

C++ Code for Deserialization

TreeNode* deserialize(string data) {
    if (data.empty()) return nullptr;
    stringstream ss(data);
    string token;
    getline(ss, token, ',');
    TreeNode* root = new TreeNode(stoi(token));
    queue<TreeNode*> q;
    q.push(root);
    while (!q.empty()) {
        TreeNode* node = q.front();
        q.pop();
        if (getline(ss, token, ',')) {
            if (token != "null") {
                node->left = new TreeNode(stoi(token));
                q.push(node->left);
            }
        }
        if (getline(ss, token, ',')) {
            if (token != "null") {
                node->right = new TreeNode(stoi(token));
                q.push(node->right);
            }
        }
    }
    return root;
}

Java Code for Deserialization

public TreeNode deserialize(String data) {
    if (data.isEmpty()) return null;
    String[] values = data.split(",");
    TreeNode root = new TreeNode(Integer.parseInt(values[0]));
    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);
    int i = 1;
    while (!queue.isEmpty()) {
        TreeNode node = queue.poll();
        if (!values[i].equals("null")) {
            node.left = new TreeNode(Integer.parseInt(values[i]));
            queue.offer(node.left);
        }
        i++;
        if (!values[i].equals("null")) {
            node.right = new TreeNode(Integer.parseInt(values[i]));
            queue.offer(node.right);
        }
        i++;
    }
    return root;
}

Time and Space Complexity Analysis

Serialize:
- Time Complexity: O(n), where n is the number of nodes in the tree, since each node is visited once.
- Space Complexity: O(n), mainly due to the storage of the string and queue.
Deserialize:
- Time Complexity: O(n), as each node value is processed once.
- Space Complexity: O(n), required for maintaining the constructed tree and the queue.

Common Mistakes to Avoid

Incorrect Use of Delimiters: Ensure consistent use of commas to separate node values and correctly handle "null" markers for absent children.
Handling Edge Cases: Manage edge inputs like empty trees or trees with a single node. Always test boundary conditions.
Infinite Loop: Prevent loops in deserialization by ensuring queue operations align correctly with data parsing.

Additional Resources and References

This comprehensive guide provides not only the solution but also the fundamental concepts needed to understand and implement serialization and deserialization in binary trees effectively.

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