[LeetCode] Problem 297 - Serialize and Deserialize Binary Tree

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Serialization is the process of converting a data structure or object into a sequence of bits so that it can be stored in a file or memory buffer, or transmitted across a network connection link to be reconstructed later in the same or another computer environment.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work. You just need to ensure that a binary tree can be serialized to a string and this string can be deserialized to the original tree structure.

Example

You may serialize the following tree:

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2
3
4
5
  1
 / \
2   3
   / \
  4   5

as “[1,2,3,null,null,4,5]”

Clarification

The above format is the same as how LeetCode serializes a binary tree. You do not necessarily need to follow this format, so please be creative and come up with different approaches yourself.

Note

Do not use class member/global/static variables to store states. Your serialize and deserialize algorithms should be stateless.

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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private int index = 0;

public String serialize(TreeNode root) {
    StringBuilder sb = new StringBuilder();
    preorder(root, sb);
    return sb.toString();
}

public TreeNode deserialize(String data) {
    String[] str = data.split(",");
    return construct(str);
}

private void preorder(TreeNode root, StringBuilder sb) {
    if (root == null) {
        sb.append("#,");
        return;
    }

    sb.append(root.val).append(",");
    preorder(root.left, sb);
    preorder(root.right, sb);
}

private TreeNode construct(String[] str) {
    if (str[index].equals("#")) {
        index++;
        return null;
    }
    
    TreeNode root = new TreeNode(Integer.parseInt(str[index++]));
    root.left = construct(str);
    root.right = construct(str);

    return root;
}