Problem

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.

Solution

public class Codec {

    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        if (root == null) return "null";
        return root.val + "," + serialize(root.left) + "," + serialize(root.right);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        List<String> values = Arrays.asList(data.split(","));
        Queue<String> queue = new LinkedList<>(values);
        return buildTree(queue);
    }
    
    private TreeNode buildTree(Queue<String> values) {
        if (values == null || values.size() == 0) return null;
        String val = values.poll();
        if (val.equals("null")) return null;
        TreeNode node = new TreeNode(Integer.valueOf(val));
        node.left = buildTree(values);
        node.right = buildTree(values);
        return node;
    }
}