\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning} 
\tikzset{
    %Define standard arrow tip
    >=stealth',
    % Define arrow style
    pil/.style={->,thick}
}

\begin{document}
  \begin{tikzpicture}
      \node (a)[vertex] at (1,1) {};
      \node (b)[vertex] at (2,5) {};
      \node (c)[vertex] at (3,3) {};
      \node (d)[vertex] at (5,4) {};
      \node (e)[vertex] at (3,6) {};
      \node (f)[vertex] at (5,6) {};
      \node (g)[vertex] at (7,6) {};
      \node (h)[vertex] at (7,4) {};
      \node (i)[vertex] at (6,2) {};
      \node (j)[vertex] at (8,7) {};
      \node (k)[vertex] at (9,5) {};
      \node (l)[vertex] at (13,6) {};
      \node (m)[vertex] at (11,7) {};
      \node (n)[vertex] at (15,7) {};
      \node (o)[vertex] at (16,4) {};
      \node (p)[vertex] at (10,2) {};
      \node (q)[vertex] at (13,1) {};
      \node (r)[vertex] at (16,1) {};
      \node (s)[vertex] at (17,4) {};
      \node (t)[vertex] at (19,6) {};
      \node (u)[vertex] at (18,3) {};
      \node (v)[vertex] at (20,2) {};
      \node (w)[vertex] at (15,4) {};

      \foreach \from/\to in {a/c,c/b,c/d,d/f,f/g,g/h,h/d,d/g,h/f,i/k,k/j,k/l,l/m,m/n,n/o,o/t,t/v,v/u,s/r,o/q,q/p,u/t}
        \draw[line width=2pt] (\from) -- (\to);
  \end{tikzpicture}
\end{document}