Serialize and Deserialize Binary Tree

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.

For example, you may serialize the following tree

1

/ \ 2 3 / \ 4 5

as "[1,2,3,null,null,4,5]", just the same as how LeetCode OJ 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.

Credits:Special thanks to @Louis1992 for adding this problem and creating all test cases.

Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 import java.util.StringTokenizer;

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuffer sb = new StringBuffer();
        serializeHelper(root, sb);
        return sb.toString();
    }

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

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


    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        StringTokenizer stringTokenizer = new StringTokenizer(data," ");
        return deserializeHelper(stringTokenizer);
    }

    private TreeNode deserializeHelper(StringTokenizer stringTokenizer) {
        if (!stringTokenizer.hasMoreElements()) {
            return null;
        }

        String currentItem = stringTokenizer.nextToken();
        if (currentItem.equals("#")) {
            return null;
        }

        TreeNode root = new TreeNode(Integer.parseInt(currentItem));
        TreeNode left = deserializeHelper(stringTokenizer);
        TreeNode right = deserializeHelper(stringTokenizer);
        root.left = left;
        root.right = right;
        return root;
    }

}

// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));