\documentclass[10pt,a4paper]{article}

\usepackage[margin=2cm]{geometry}
\usepackage{multicol}

\let\assignment7

\usepackage{enumitem}
\setenumerate[1]{label=\assignment.\arabic*.}
\setenumerate[2]{label=\textit{\alph*})}

% textcomp package is not available everywhere, and we only need the Copyright symbol
% taken from http://tex.stackexchange.com/a/1677/23992
\DeclareTextCommandDefault{\textregistered}{\textcircled{\check@mathfonts\fontsize\sf@size\z@\math@fontsfalse\selectfont R}}

\usepackage{fancyhdr}
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}
\fancyhead{}
\fancyfoot[C]{Copyright {\textcopyright} 2015 Camil Staps}
\pagestyle{fancy}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\DeclareMathAlphabet{\pazocal}{OMS}{zplm}{m}{n}

\usepackage{tikz}
\usepackage{array}
\usepackage{caption}
\usepackage[hidelinks]{hyperref}
\usepackage{url}

\parindent0pt

\title{Algoritmen en Datastructuren - assignment \assignment}
\author{Camil Staps\\\small{s4498062}}

\begin{document}

\maketitle
\thispagestyle{fancy}

\begin{multicols}{2}

\begin{enumerate}
    \item The initial tree would be:
        
        \begin{minipage}{\linewidth}
            \centering
            \vskip10pt
            \begin{tikzpicture}[every node/.style={draw,circle,minimum width=2em},
                                level 1/.style={sibling distance=9em},
                                level 2/.style={sibling distance=5em},
                                level 3/.style={sibling distance=3em},
                                font={\footnotesize}]
                \node {9}
                    child { node {5}
                        child { node {4} }
                        child { node {7}
                            child[missing] { node {} }
                            child { node {8} }
                        }
                    }
                    child { node {12}
                        child { node {10}
                            child[missing] { node {} }
                            child { node {11} }
                        }
                        child { node {18} }
                    };
            \end{tikzpicture}
        \end{minipage}\\

        After removing $9$:

        \begin{minipage}{\linewidth}
            \centering
            \vskip10pt
            \begin{tikzpicture}[every node/.style={draw,circle,minimum width=2em},
                                level 1/.style={sibling distance=9em},
                                level 2/.style={sibling distance=5em},
                                level 3/.style={sibling distance=3em},
                                font={\footnotesize}]
                \node {10}
                    child { node {5}
                        child { node {4} }
                        child { node {7}
                            child[missing] { node {} }
                            child { node {8} }
                        }
                    }
                    child { node {12}
                        child { node {11} }
                        child { node {18} }
                    };
            \end{tikzpicture}
        \end{minipage}\\

        After removing $5$:

        \begin{minipage}{\linewidth}
            \centering
            \vskip10pt
            \begin{tikzpicture}[every node/.style={draw,circle,minimum width=2em},
                                level 1/.style={sibling distance=9em},
                                level 2/.style={sibling distance=5em},
                                font={\footnotesize}]
                \node {10}
                    child { node {7}
                        child { node {4} }
                        child { node {8} }
                    }
                    child { node {12}
                        child { node {11} }
                        child { node {18} }
                    };
            \end{tikzpicture}
        \end{minipage}

    \item \begin{enumerate}
            \item %todo
            \item For a node $v$:

                \begin{itemize}
                    \item If $v$ is nil, return $0$.
                    \item If $v$ is a leaf node, return $1$.
                    \item If $v$ has children, return the sum of the number of leaves of the children.
                \end{itemize}

                Note that this works on any tree.
        \end{enumerate}

    \item %todo

    \item %todo

    \item %todo

\end{enumerate}

\end{multicols}

\end{document}