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Introduction to Serialize and Deserialize Tree Pattern

The Serialize and Deserialize Tree Pattern is commonly used to transform a binary tree into a different format, such as a string or an array, for storage or transmission. Once serialized, the tree can be deserialized to recover its original structure. This pattern is widely applied in systems that require saving or sending binary trees across different platforms or networks. Proper serialization and deserialization ensure that the structure and values of the tree are preserved correctly.

Mastering this pattern is essential for solving problems where binary trees need to be stored or shared. Handling both null nodes and maintaining the correct traversal order during serialization and deserialization is the key to preserving the tree's structure.

Now, let's understand the serialize and deserialize tree pattern with an example problem.

Problem Statement

Given a binary tree, your task is to create two functions.

One for serializing the tree into a string format and another for deserializing the string back into the tree.

The serialized string should retain all the tree nodes and their connections, allowing for reconstruction without any loss of data.

Examples

Example 1:

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Example 2:

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Solution

To serialize a binary tree, we will traverse it in a pre-order fashion (root-left-right) and generate a string representation. When we encounter a null value, we'll represent it with a special character, say "X". The serialized string will have each value separated by a comma.

For deserialization, we will split the string on commas and use a queue to help in the reconstruction. We'll take the front of the queue and check if it's "X". If it is, then it's a null node, otherwise, we create a new node with the value. The same approach applies recursively for the left and right children.

Step-by-Step Algorithm

  1. Serialization:
    • Start from the root.
    • If the current node is null, append "X," to the result string.
    • If not null, append its value and a comma to the result string.
    • Recursively serialize the left subtree.
    • Recursively serialize the right subtree.
  2. Deserialization:
    • Split the string by comma to get a list of nodes.
    • Use a queue to facilitate tree reconstruction.
    • For each value in the list:
      • If it's "X", return null.
      • Otherwise, create a new node.
      • Recursively deserialize the left and right children.

Algorithm Walkthrough:

Serialization

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Deserialization:

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Code

java
import java.util.*;

// Definition for a binary tree node.
//    public static class TreeNode {
//     int val;
//     TreeNode left;
//     TreeNode right;
//     TreeNode(int x) { val = x; }
//     TreeNode(int x, TreeNode left, TreeNode right) {  // Overloaded constructor
//         this.val = x;
//         this.left = left;
//         this.right = right;
//     }
// }

public class Solution {

  // Encodes a tree to a single string using a pre-order traversal.
  public String serialize(TreeNode root) {
    StringBuilder sb = new StringBuilder();
    serializeHelper(root, sb);
    return sb.toString();
  }

  private void serializeHelper(TreeNode node, StringBuilder sb) {
    // If the current node is null, append "X" to the result string.
    if (node == null) {
      sb.append("X,");
    } else {
      // Append the node value and then serialize left and right subtrees.
      sb.append(node.val + ",");
      serializeHelper(node.left, sb);
      serializeHelper(node.right, sb);
    }
  }

  // Decodes your encoded data to tree.
  public TreeNode deserialize(String data) {
    Queue<String> queue = new LinkedList<>(Arrays.asList(data.split(",")));
    return deserializeHelper(queue);
  }

  private TreeNode deserializeHelper(Queue<String> queue) {
    // Extract the value from the queue.
    String val = queue.poll();
    if (val.equals("X")) return null;
    // Create a node and then recursively deserialize left and right children.
    TreeNode node = new TreeNode(Integer.parseInt(val));
    node.left = deserializeHelper(queue);
    node.right = deserializeHelper(queue);
    return node;
  }

  public static void main(String[] args) {
    Solution solution = new Solution();
    TreeNode testTree = new TreeNode(
      1,
      new TreeNode(2),
      new TreeNode(3, new TreeNode(4), new TreeNode(5))
    );
    String serialized = solution.serialize(testTree);
    TreeNode deserialized = solution.deserialize(serialized);
    System.out.println("Serialized: " + serialized);
    System.out.println(
      "Deserialized (Serialized again for verification): " +
      solution.serialize(deserialized)
    );
  }
}

Complexity Analysis

Time Complexity

Space Complexity

Now, let's start solving a few more problems on Serialize and Deserialize Tree Pattern.

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