297. Serialize and Deserialize Binary Tree

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https://leetcode.com/problems/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.
Example: 
You may serialize the following tree:

    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.
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Intuition
Any standard traversal in order, pre order, post order can be used. We choose in order for the solution - easy on recursion
Note - 
  1. null indicator - any character which does not represent a string
  2. delimiter - any character to separate numbers which may be of multiple digits

On Deserialize - pickup a data structure which offers FIFO, and keep reducing the data structure size to avoid dealing with position pointers arithmetic - Queue is a good choice
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Time - O(n)
Space - O(n)
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Related problems
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public class Codec {
// Encodes a tree to a single string.
public String serialize(TreeNode root) {
if (root == null) {
return "#";
}
return root.val + "," + serialize(root.left) + "," + serialize(root.right);
}
// Decodes your encoded data to tree.
int i;
public TreeNode deserialize(String data) {
i = 0;
String[] tokens = data.split(",");
return dfs(tokens);
}
private TreeNode dfs(String[] tokens) {
if (i == tokens.length || tokens[i].equals("#")) {
i = i + 1;
return null;
}
TreeNode node = new TreeNode(Integer.parseInt(tokens[i++]));
node.left = dfs(tokens);
node.right = dfs(tokens);
return node;
}
}
// Your Codec object will be instantiated and called as such:
// Codec codec = new Codec();
// codec.deserialize(codec.serialize(root));
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