# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Codec:

    def serialize(self, root):
        """Encodes a tree to a single string.

        :type root: TreeNode
        :rtype: str
        """
        if not root:
            return None
        arr = []
        queue = collections.deque([root])
        while queue:
            length = len(queue)
            for _ in range(length):
                popped = queue.popleft()
                if popped:
                    arr.append(str(popped.val))
                    queue.append(popped.left)
                    queue.append(popped.right)
                else:
                    arr.append("null")
        return ",".join(arr)



    def deserialize(self, data):
        """Decodes your encoded data to tree.

        :type data: str
        :rtype: TreeNode
        """
        if not data:
            return None
        arr = data.split(",")
        root = TreeNode(int(arr[0]))
        queue = collections.deque([root])
        i = 1
        while queue:
            popped = queue.popleft()
            if arr[i] != 'null':
                popped.left = TreeNode(int(arr[i]))
                queue.append(popped.left)
            i += 1
            if arr[i] != 'null':
                popped.right = TreeNode(int(arr[i]))
                queue.append(popped.right)
            i += 1
        return root




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