(root)
return root
type root: root class
type root: root class
type root : root class
def bin_tree_to_list(root):
    """
    type root: root class
    """
    if not root:
        return root
    root = bin_tree_to_list_util(root)
    while root.left:
        root = root.left
    return root
def bin_tree_to_list ( root ) : if not root : return root root = bin_tree_to_list_util ( root ) while root . left : root = root . left return root