flatten binary tree to linked list

# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class (object):
    def flatten(self, root):
        """
        :type root: TreeNode
        :rtype: void Do not return anything, modify root in-place instead.
        """
        self.flat(root)
    
    def flat(self, root):
        # it is like preorder traversal
        if not root:
            return None
        right = root.right
        root.right = self.flat(root.left)
        root.left = None
        head = root
        while head.right:
            head = head.right
        head.right = self.flat(right)
        return root