def height(t):
    """
    Return 1 + length of the longest path of t.
    @param BinaryTree t: binary tree to find the height of
    @rtype: int
    >>> t = BinaryTree(13)
    >>> height(t)
    1
    >>> height(BinaryTree(5, BinaryTree(3), BinaryTree(8, BinaryTree(7))))
    3
    """
    if t is None:
        return 0
    else:
        return 1 + max(height(t.left), height(t.right))