Vertically align text in two columns

Question

I am writing in two columns. I am trying to align the text vertically such that it does not add extra spaces. Happy for the content to be finished before the end of the page.
This is what I want:

The class for this one is dndbook

\documentclass[letterpaper,twocolumn,openany,nodeprecatedcode]{dndbook}
\usepackage{listings}
\usepackage{shortvrb}
\usepackage{stfloats}
\MakeShortVerb{|}
\lstset{%
  basicstyle=\ttfamily,
  language=[LaTeX]{TeX},
  breaklines=true,
}
\begin{document}
\frontmatter
\maketitle
\tableofcontents

This is what I have:

I am using \raggedbottom and extarticle class:

\documentclass[twoside,twocolumn,10pt]{extarticle}
\raggedbottom
\begin{document}
\tableofcontents
\pagebreak
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\section{A section}
\end{document}

Answer 1

Rather than using the twocolumn option for the document class, consider using the multicol package (\usepackage{multicol}) with a single \begin{multicol}{2} environment surrounding the table of contents:

\begin{multicols}{2}
\tableofcontents
\end{multicols}
\pagebreak

(You can start another one on the next page if need be.)

See the package documentation.