\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{shapes, calc} 
\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=20pt,inner sep=0pt]

\begin{document}
\begin{preview}
\begin{tikzpicture}
    \node[vertex] {+}
        child { 
            node[vertex] {+}
            child { 
                node[vertex] {+} 
                child { node[vertex] {i} }
                child { node[vertex] {++i} }
            }
            child { node[vertex] {i++} }
        } child {
            node[vertex] {++i}
        }
    ;
\end{tikzpicture}
\end{preview}
\end{document}