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.

/**
 * 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 "";
        StringBuilder sb = new StringBuilder();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            sb.append(node.val);
            if (node.left != null) {
                sb.append("l");
                queue.add(node.left);
            }
            if (node.right != null) {
                sb.append("r");
                queue.add(node.right);
            }
            sb.append("#");
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.isEmpty()) return null;
        LinkedList<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(0);
        int val = 0;
        int sign = 1;
        TreeNode node = root;
        for (int i = 0; i < data.length(); i++) {
            if (data.charAt(i) == '-') {
                sign = -1;
            } else if (Character.isDigit(data.charAt(i))) {
                val *= 10;
                val += data.charAt(i) - '0';
            } else if (data.charAt(i) == 'l') {
                node.left = new TreeNode(0);
                queue.add(node.left);
            } else if (data.charAt(i) == 'r') {
                node.right = new TreeNode(0);
                queue.add(node.right);
            } else {
                node.val = sign * val;
                val = 0;
                sign = 1;
                if (!queue.isEmpty())
                    node = queue.poll();
            }
        }
        return root;
    }
}

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

/**
 * 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 "";
        StringBuilder sb = new StringBuilder();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            sb.append(node.val);
            if (node.left != null && node.right == null) {
                sb.append("l");
                queue.add(node.left);
            } else if (node.right != null && node.left == null) {
                sb.append("r");
                queue.add(node.right);
            } else if (node.right != null && node.right != null) {
                sb.append('b');
                queue.add(node.left);
                queue.add(node.right);
            } else {
                sb.append("e");
            }
        }
        return sb.toString();
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data.isEmpty()) return null;
        LinkedList<TreeNode> queue = new LinkedList<>();
        TreeNode root = new TreeNode(0);
        int val = 0;
        int sign = 1;
        TreeNode node = root;
        for (int i = 0; i < data.length(); i++) {
            if (data.charAt(i) == '-') {
                sign = -1;
            } else if (Character.isDigit(data.charAt(i))) {
                val *= 10;
                val += data.charAt(i) - '0';
            } else {
                if (data.charAt(i) == 'l') {
                    node.left = new TreeNode(0);
                    queue.add(node.left);
                } else if (data.charAt(i) == 'r') {
                    node.right = new TreeNode(0);
                    queue.add(node.right);
                } else if (data.charAt(i) == 'b') {
                    node.left = new TreeNode(0);
                    node.right = new TreeNode(0);
                    queue.add(node.left);
                    queue.add(node.right);
                }
                node.val = sign * val;
                val = 0;
                sign = 1;
                if (!queue.isEmpty())
                    node = queue.poll();
            }
        }
        return root;
    }
}

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