class Codec:

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

        :type root: TreeNode
        :rtype: str
        """

        this_level = [root]
        out = []

        if not root:
            return ','.join(out)

        while this_level:
            next_level = []
            for item in this_level:
                if item is not "#":
                    if item.left: next_level.append(item.left)
                    else: next_level.append("#")
                    if item.right: next_level.append(item.right)
                    else: next_level.append("#")
                    out.append(str(item.val))
                else:
                    out.append("#")
            this_level = next_level
        return ','.join(out)