\documentclass{standalone}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{parskip}
\usepackage{marvosym} %Lightning symbol
\usepackage[usenames,dvipsnames]{color}
\usepackage[hidelinks]{hyperref}
\renewcommand*{\familydefault}{\sfdefault}

\usepackage{bbm} %For \mathbbm{1}
%\usepackage{bbold}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
    \draw[gray] (0,0) -- (10,0);
    \draw[gray,dotted] (0,2) -- (10,2);
    \draw[gray] (0,2) arc (90:270:1);
    \draw[gray] (10,0) arc (-90:90:1);
    \foreach \x in {0,...,10} {
        \draw[fill] (\x,0) circle (0.04);
    }
    \draw[fill] (11,1) circle (0.04);
    \draw[fill] (-1,1) circle (0.04);

    \foreach \x in {1,2,4,7,9} {
        \draw[fill,red] (\x,0) circle (0.09);
    }
    \foreach \x in {3,5,6,8} {
        \draw[fill,blue] (\x-0.07,0-0.07) rectangle +(0.14,0.14);
    }

    \draw[<->] (1,-0.5) -- (9,-0.5);
    \draw (5,-0.9) node {$\mathcal{D}(C)$};

    \draw[<->] (4.9,0.3) -- (6.1,0.3);
    \draw (5.5,0.7) node {$\mathrm{maxgap}(C)$};

    \draw[fill,red] (1,-2) circle (0.09);
    \draw (1.5,-2) node {$C$};
    %\draw[fill] (3,-2) circle (0.04);
    %\draw (4.5,-2) node {non-slots $[n]\setminus C$};
    \draw[fill,blue] (7.3-0.07,-2-0.07) rectangle +(0.14,0.14);
    \draw (8,-2) node {$C_{\mathrm{gaps}}$};
\end{tikzpicture}

\end{document}