Drawing AND/OR trees in Tikz

Question

Is there a simple way to achieve this type of graph?

Obviously a lot neater than this... But the important feature is the join between the two children of P.

Answer 1

You should study pgfmanual (v2.10 as of mine) for trees library (sections 18.Making Trees Grow, 53.Tree Library) and placing nodes on a line or curve (sections 16.8 and 16.9).

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{trees}
\begin{document}
\begin{tikzpicture}[level distance=1.5cm, grow=down,
    every node/.style={draw, circle, thin},
    edge from parent/.style={-latex, thick, draw}
]
\node (P) {P}
    child {node (Q) {Q}
        child {node (T) {T}}
        child {node (U) {U}}
    }
    child {node (R) {R}}
    child {node (S) {S}};

\path (P) -- coordinate[midway] (PQ) (Q);
\path (P) -- coordinate[midway] (PR) (R);

\draw (PQ) to[bend right=22] (PR);
\end{tikzpicture}
\end{document}