\pstree and crossed branches

Question

Still a TeX newbie. I'm working with PSTricks pstree, and wish a tree whose branches cross:

        I
       / \
      G   H
     / \ / \
    F   /   E
   / \ / \ 
  /   /   \ 
 /   / \   \ 
A   B   C   D 

If it's not clear from the ASCII picture,

F -> A C
G -> F D
H -> B E

A < B < C < D.

My usual tree code looks like:

\pstree{ I }{
  \pstree{ G }{ ... }
  \pstree{ H }{ ... }
}

Answer 1

You could do this using TikZ. You can use [missing] to create "empty" branches that maintain the correct spacing but don't draw any objects. By doubling the level distance and sibling distance for the third level compared to the first two levels, the branches will cross while the angles of the branches are constant all throughout the tree (I'm not sure if that's what you actually wanted):

\documentclass{article}
\usepackage{tikz}


\begin{document}
\begin{tikzpicture}[
    level 3/.style={
        level distance=30mm,
        sibling distance=30mm
    }
]
\node {I}
child {
    node {G}
    child {
        node {F}
        child {
            node {A}
        }
        child {
            node {C}
        }
    }
    child {
        child [missing]
        child{
            node {D}
        }
    }
}
child {
    node {H}
    child {
        child {
            node {B}
        }
        child [missing] 
    }
    child {
        node {E}
    }
};
\end{tikzpicture}
\end{document}

---

Alternatively, you can just specify the level distance and sibling distance for individual child branches to get the desired placement:

\documentclass{article}
\usepackage{tikz}


\begin{document}
\begin{tikzpicture}[
    level distance=15mm,
    sibling distance=15mm,
    level 3/.style={
    }
]
\node {I}
child {
    node {G}
    child {
        node {F}
        child {
            node {A}
        }
        child {
            node {C}
        }
    }
    child [level distance=30mm, sibling distance=30mm] {
        node {D}
    }
}
child {
    node {H}
    child [level distance=30mm, sibling distance=45mm] {
        node {B}
    }
    child {
        node {E}
    }
};
\end{tikzpicture}
\end{document}

Answer 2

Here's a solution using pst-tree. You can tweak the parameters treesep and perhaps add a few more null nodes \Tn to get the spacing the way you want.

To get the lines to cross, I (perhaps cheated and) gave the relevant nodes names, and then connected them using an \ncline.

\documentclass{article}

\usepackage{pst-tree}

\begin{document}

\pstree[nodesep=3pt]{\TR{I}}{%
        \pstree{\TR[name=G]{G}}%
                {%
                    \pstree{\TR{F}}%
                    {\TR{A}\TR[name=B,edge=none]{B}\TR{C}\TR[name=D,edge=none]{D}}%
                        \Tn\Tn%
                }%
        \pstree{\TR[name=H]{H}}%
               {\Tn\TR{E}}%
    }% end the tree
    \ncline[nodesep=3pt]{H}{B}
    \ncline[nodesep=3pt]{G}{D}

\end{document}