\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}

\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc}
\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=18pt,inner sep=0pt]


\begin{document}
\begin{preview}
\begin{tikzpicture}[very thick,level/.style={sibling distance=70mm/#1}]
\node [vertex] (r){$17$}
  child {
    node [vertex] (a) {$9$}
    child {
      node [vertex] {$3$}
      child {
        node [vertex] {$-3$}
        child {node [vertex] {$-4$}}
        child {
            node [vertex] {$2$}
            child {
                node [vertex] {$2$}
            }
        }
      }
      child {
        node [vertex] {$8$}
        child {
            node [vertex] {$6$}
            child {
                node [vertex] {$5$}
            }
            child {
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    }
    child {
      node [vertex] {$11$}
      child {
        node [vertex] {$17$}
          child {
            node [vertex] {$17$}
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  }
  child {
    node [vertex] {$19$}
    child {
      node [vertex] {$20$}
    }
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\end{tikzpicture}
\end{preview}
\end{document}
