pstree: How can I force a tree node to one side?

I'm drawing some binary search trees using pstricks and pstree, but I don't see anything in the docs to push a node to one side. In a binary search tree, every child must be either right or left, but if there's only one child then pstree will hang it straight down.

   *
/ \
*   *
/ \  |


should be

   *
/ \
*   *
/ \   \

-

Putting a nil node is not tricky but rather good maths. Adding manually some space would be ugly.

In pstree you manipulate (ordered) n-ary trees so there is no notion of left or right child node. You can only give a list of child nodes (ordered).

Think of how you will implement binary trees (t := () | (t, t)) with ordered n-ary trees (t := list of trees). The two following binary trees are different.

  *          *
/     !=     \
*              *


While they are equal as n-ary trees. You need to explicitly encode the skip of the left child node to simulate that in n-ary trees.

The pstree creators proposed you a distinguished \Tn node (nil node) for that purpose.

\pstree{\TR{a}}{
\Tn
\TR{b}
}


Just like when a missing information is an information.

-

I resorted to inserting invisible nodes ({\psset{linestyle=none}\TR{}}) into the tree, which pushes the other node towards one side. It worked for what I needed, but is a bit hackish and behaves like an invisble node, not as a well-defined offset to one side. (For example, in the rendered image, it would be a hassle to shift that 4 further right where it should be.)

\pstree[levelsep=25pt]{\Tcircle{10}}
{
\pstree{\Tcircle{1}}
{
{\psset{linestyle=none}\TR{}}
\pstree{\Tcircle{5}}
{
\Tcircle{4}
{\psset{linestyle=none}\TR{}}
}
}
\pstree{\Tcircle{16}}
{
{\psset{linestyle=none}\TR{}}
\pstree{\Tcircle{17}}
{
{\psset{linestyle=none}\TR{}}
\Tcircle{21}
}
}
}


Gives

  10
/  \
1    16
\     \
5     17
/        \
4          21


-

use \Tn for a "null node" ...

\documentclass{article}
\usepackage{pst-tree}
\begin{document}

\pstree[levelsep=25pt]{\Tcircle{10}}{
\pstree{\Tcircle{1}}{
\Tn
\pstree{\Tcircle{5}}{\Tcircle{4}{\Tn}}
}
\pstree{\Tcircle{16}}{
\Tn
\pstree{\Tcircle{17}}{
\Tn
\Tcircle{21}
}
}
}

\end{document}


-