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In an article environment, how do I bring the tikzpicture to the center of the page. begin{center} and end{center} do not work.

documentclass{article}

usepackage{tikz}

usetikzlibrary{positioning}

usetikzlibrary{arrows}

usetikzlibrary{backgrounds}

usetikzlibrary{fit}

usetikzlibrary{decorations.pathmorphing}

usetikzlibrary{calc}



tikzset{squarenode/.style = {

    shape  = rectangle,

    fill   = gray!50,

    draw   = black,

    thick,

    minimum height = 1cm,

    minimum width  = 1cm

}}



tikzset{circlenode/.style = {

    shape  = circle,

    fill   = blue!20,

    draw   = blue,

    thick,

    minimum size = 1cm

}}



tikzset{redcirclenode/.style = {

    shape  = circle,

    fill   = red!20,

    draw   = red,

    thick,

    minimum size = 1cm

}}



tikzset{help lines/.style=very thin}

tikzset{My Grid/.style={help lines,color=blue!50}}



begin{document}

begin{center}

begin{tikzpicture}

  [>=stealth']

  %draw[My Grid] (-5,-5) grid (15,15);

  node (r1) at (-4,4) [squarenode] {};

  node[right=4cm of r1] (r2) [squarenode] {};

  node[below=3cm of r1] (r3) [squarenode] {};

  node[right=4cm of r3] (r4) [squarenode] {};

  node[right=1.5cm of r1] (s2) [circlenode] {};

  node[above=1cm of s2] (s1) [circlenode] {}; %s1

  node[below=1cm of s2] (s3) [circlenode] {}; %s3

  node[above=1ex of s3,red] {$s leq 3$};

  node[right=1.5cm of r3] (s4) [circlenode] {};

  node[below=1cm of s4] (s5) [circlenode] {}; %s5

  draw[fill=black] (s1.center) circle [radius=0.1cm];

  draw[fill=black] (s5.center) circle [radius=0.1cm];

  draw[->] (r1) -- (s2);

  draw[->] (s2) -- (r2);

  draw[->] (r3) -- (s4);

  draw[->] (s4) -- (r4);

  draw[->] (s1) to [out=180,in=90] (r1);

  draw[->] (r1) to [out=-90,in=180] (s3);

  draw[->] (s3) to [out=0,in=-90] (r2);

  draw[->] (r2) to [bend right=45] node[auto,swap] {2} (s1);

  draw[->] (r3) to [out=90,in=180] (s3);

  draw[->] (s3) to [out=0,in=90] (r4);

  draw[->] (r4) to [bend left=45] node[auto] {2} (s5);

  draw[->] (s5) to [bend left=45] (r3);

  begin{scope}[on background layer]

    node [fill=gray!15,rounded corners,fit=(s1) (s3) (s5) (r1) (r2)] (R1) {};

  end{scope}



  begin{scope}[xshift=10cm]

    node (r1) at (-4,4) [squarenode] {};

    node[right=4cm of r1] (r2) [squarenode] {};

    node[below=3cm of r1] (r3) [squarenode] {};

    node[right=4cm of r3] (r4) [squarenode] {};

    node[right=1.5cm of r1] (s2) [circlenode] {};

    node[above=1cm of s2] (s1) [circlenode] {}; %s1

    node[below left=1.35cm and 0cm of s2,] (s3) [redcirclenode] {}; %s3

    node[right=1.5cm of r3] (s4) [circlenode] {};

    node[below=1cm of s4] (s5) [circlenode] {}; %s5

    node[right=0.5cm of s3] (s6) [redcirclenode] {}; %s6

    node[left=1ex of s3,red] {$s$};

    node[right=1ex of s6,red] {$bar{s}$};

    draw[fill=black] (s1.center) circle [radius=0.1cm];

    draw[fill=black] (s5.center) circle [radius=0.1cm];

    draw[fill=black] ($(s6.center)-(0.23,0.15)$) circle [radius=0.1cm];

    draw[fill=black] ($(s6.center)-(0,-0.23)$) circle [radius=0.1cm];

    draw[fill=black] ($(s6.center)+(0.23,-0.15)$) circle [radius=0.1cm];

    draw[->] (r1) -- (s2);

    draw[->] (s2) -- (r2);

    draw[->] (r3) -- (s4);

    draw[->] (s4) -- (r4);

    draw[->] (s1) to [out=180,in=90] (r1);

    draw[->] (r1) to (s3);

    draw[->] (s6) to (r1);

    draw[->] (s3) to (r2);

    draw[->] (r2) to (s6);

    draw[->] (r2) to [bend right=45] node[auto,swap] {2} (s1);

    draw[->] (r3) to (s3);

    draw[->] (s6) to (r3);

    draw[->] (s3) to (r4);

    draw[->] (r4) to (s6);

    draw[->] (r4) to [bend left=45] node[auto] {2} (s5);

    draw[->] (s5) to [bend left=45] (r3);

        begin{scope}[on background layer]

            node [fill=gray!15,rounded corners,fit=(s1) (s3) (s5) (r1) (r2)] (R2) {};

        end{scope}

  end{scope}



  draw[shorten >=1mm,-to,thick,decorate,decoration={snake,amplitude=.4mm,segment length=2mm,pre=moveto,pre length=1mm,post length=2mm}] (R1) -- (R2) node [above=1mm,midway,text width=3cm,align=center] {replacement of the textcolor{red}{capacity} by textcolor{red}{two places}};



end{tikzpicture}  

end{center}

end{document}