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.

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.

思路

1. Serialization
   1. use bfs to traverse the tree. need a queue
   2. a stringbuider
   3. put root to the queue
   4. when the queue is not empty
      1. add ','
      2. check left node of current node
         1. null then add '#'
         2. not null then add val
      3. check right node of current node the way as the left node
2. Deserialization
   1. split the data with ','
   2. create root node with the first element
   3. create left node and right node and connect to root node level by level, so need a queue
   4. maintain an index for the array from splitting string
   5. put root node to the queue
   6. while the queue is not empty
      1. check left node and right node
         1. not #, then construct, connect, enqueue
         2. #, do nothing

    public String serialize(TreeNode root) {
        if (root == null) return "";
        StringBuilder sb = new StringBuilder();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        sb.append(root.val);
        while (!queue.isEmpty()) {
            TreeNode curt = queue.poll();
            sb.append(',');
            if (curt.left == null) {
                sb.append("#");
            } else {
                sb.append(curt.left.val);
                queue.offer(curt.left);
            } 
            sb.append(',');
            if (curt.right == null) {
                sb.append('#');
            } else {
                sb.append(curt.right.val);
                queue.offer(curt.right);
            }
        }
        return sb.toString();
    }

    public TreeNode deserialize(String data) {
        if (data == null || data.length() == 0) return null;
        String[] arr = data.split(",");
        TreeNode root = new TreeNode(Integer.parseInt(arr[0]));
        int index = 1;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode curt = queue.poll();
            if (!"#".equals(arr[index])) {
                TreeNode left = new TreeNode(Integer.parseInt(arr[index]));
                curt.left = left;
                queue.offer(left);
            }
            index++;
            if (!"#".equals(arr[index])) {
                TreeNode right = new TreeNode(Integer.parseInt(arr[index]));
                curt.right = right;
                queue.offer(right);
            }
            index++;
        }
        return root;
    }
}