Implement Binary Tree Serialization and Deserialization

Implement Binary Tree Serialization and Deserialization

Problem Description
Design an algorithm to implement serialization and deserialization of a binary tree. Serialization is the process of converting a binary tree into a string or byte sequence so that it can be stored in a file or memory buffer, or transmitted over a network connection. Deserialization is the process of reconstructing the original binary tree structure from the string or byte sequence. You need to ensure that the binary tree can be serialized into a string and that the string can be deserialized back into the original tree structure.

Solution Approach
The core of this problem lies in choosing a traversal method to represent the binary tree and designing a format to distinguish node values from null pointers. We will use preorder traversal (root-left-right) because it can naturally reconstruct the tree structure during deserialization. The key point is to represent null nodes, usually marked with special symbols (such as "null" or "#").

Detailed Steps

1. Serialization Process (Binary Tree → String)

  • Use preorder traversal: visit the root node first, then recursively traverse the left subtree, and finally the right subtree.
  • For null nodes, we use the special marker "null".
  • Separate nodes with commas to form a clear sequence.

Specific steps:

  1. If the current node is null, append "null," to the string.
  2. If the current node is not null, convert the node value to a string and append a comma.
  3. Recursively process the left subtree.
  4. Recursively process the right subtree.

Example: Binary tree [1,2,3,null,null,4,5] is serialized as "1,2,null,null,3,4,null,null,5,null,null,"

2. Deserialization Process (String → Binary Tree)

  • Split the serialized string by commas into a list of nodes.
  • Reconstruct the binary tree in the order of preorder traversal:
    1. Take an element from the front of the list.
    2. If it is "null", return a null node.
    3. Otherwise, create a new node with that value.
    4. Recursively construct the left subtree.
    5. Recursively construct the right subtree.

3. Code Implementation Details

  • Use StringBuilder during serialization to improve string concatenation efficiency.
  • Use a queue or iterator to process nodes in order during deserialization.
  • Pay attention to edge cases (empty tree).

4. Complexity Analysis

  • Time complexity: O(n), each node is visited exactly once.
  • Space complexity: O(n), for recursive stack space and storage of serialization results.

5. Other Implementation Methods

  • Level-order traversal (BFS) can also be used for serialization.
  • For sparse trees with many null nodes, consider more compact representation methods.
  • In practical applications, issues such as number conversion format and precision may need to be considered.

Key Points

  1. Null nodes must be explicitly marked; otherwise, the tree structure cannot be uniquely determined.
  2. The choice of delimiter should ensure it does not conflict with node values.
  3. The recursive nature of preorder traversal makes the code implementation concise and intuitive.
  4. During deserialization, pay attention to maintaining the global index or queue.

With this approach, we can completely save and restore the structure of the binary tree, ensuring that the serialization and deserialization processes are reversible.