How can I draw a tree like this?

Question

I cannot figure out how to add the boxes with numbers in them.

I've tried using forest and tikz but was not able to do this. I was able to draw a tree structure easily, but that's it.

Answer 1

Possible solution with forest:

\documentclass[margin=3mm]{standalone}
\usepackage{forest}
\usetikzlibrary{arrows.meta,
                shapes.multipart}
\newcommand\mpn[2]{\nodepart{one}  #1
                   \nodepart{two}  #2}
\newcommand\rsp{rectangle split parts}
\begin{document}
\tikzset{every label/.style = {font=\footnotesize\sffamily\bfseries}}
    \begin{forest}
for tree = {
    rectangle split,
    draw,
    %math content,
    %
    parent anchor=south,
    child anchor=north,
    edge = {semithick},
    l sep=12mm,
    s sep=11mm,
            }
[,coordinate
    [, coordinate
        [123,\rsp=1, label=below:L1]
        [,coordinate
            [123,\rsp=1,label=below:L2]
            [{\mpn{126}{346}},\rsp=2,label=below:L3]
        ]
    ]
    [, coordinate
        [{\mpn{126}{346}},\rsp=2,label=below:L4]
        [245,\rsp=1, label=below:L5]
    ]
]
    \end{forest}
\end{document}

Answer 2

This time I decided to go for a more "minimalist" way and keep the tikz code to a minimum.

Output

Code

\documentclass[margin=10pt, tikz]{standalone}
\usepackage{forest}
\tikzset{
    fornode/.style={% node definition
        draw, text width=1.2cm, line width=.5pt, inner xsep=0, align=center, execute at begin node=\setlength{\baselineskip}{10pt}},
    mlabel/.style={% our label
        label={[font=\sffamily\bfseries, anchor=north]below:#1}
    }
}
\forestset{%
    empty nodes/.style={% allows for straight edges when the node is empty
        for tree={calign=fixed edge angles},
        delay={where content={}{shape=coordinate,for siblings={anchor=north}}{}}
    }
}
\newcommand\msep{% So we don't have to write it every time
    \\vspace{-6pt}\rule{\textwidth}{.5pt}\
}
\begin{document}
\begin{forest}
    for tree={%
        fornode,
        parent anchor=south,
        child anchor=north,
        s sep=1cm
    }
    [,draw=none
        [,empty nodes 
            [134, mlabel=L1] 
            [,empty nodes
                [123, mlabel=L2]
                [126\msep 346, mlabel=L3] 
            ]
        ]
        [,empty nodes
            [234\msep 456, mlabel=L4]
            [245, mlabel=L5]
        ]
    ]
\end{forest}
\end{document}

Answer 3

You can implement the terminal nodes as simple tables. Depending on how you want the nodes to look you can align the tables using the [b] parameter (as shown on the L2/L3 nodes) or not (as in the L4/L5 nodes). If you do use the [b] parameter, you must wrap the entire tabular environment in {...} to protect the [ and ] from forest's tree parsing algorithm. The forest package doesn't deal with unlabelled nodes in trees, so if you have such nodes as in your tree, there's always a bit of a compromise. The built-in style nice empty nodes gives an unappealing result, so I've used a fairly nice empty nodes style instead.

\documentclass{article}
\usepackage[linguistics]{forest}
\forestset{fairly nice empty nodes/.style={
            delay={where content={}{shape=coordinate,for siblings={
                  {anchor=north}}}{}}
}
}
\begin{document}
\begin{forest}fairly nice empty nodes 
[
    [[\begin{tabular}{|c|}\hline1 3 4\\hline\multicolumn{1}{c}{\textbf{L1}}\end{tabular}
    ]
    [   [{\begin{tabular}[b]{|c|}\hline1 2 3\\hline\multicolumn{1}{c}{\textbf{L2}}\end{tabular}} % if you use the [b] argument you must wrap the whole tabular in {}
        ]
        [{\begin{tabular}[b]{|c|}\hline1 2 6\\hline3 4 6\\hline\multicolumn{1}{c}{\textbf{L3}}\end{tabular}}
        ]
    ]
    ]
    [[\begin{tabular}{|c|}\hline1 2 6\\hline3 4 6\\hline\multicolumn{1}{c}{\textbf{L4}}\end{tabular}
    ]
    [\begin{tabular}{|c|}\hline2 4 5\\hline\multicolumn{1}{c}{\textbf{L5}}\end{tabular}
    ]
    ] 
]
\end{forest}
\end{document}