Main file which imports the template in the file game-tree.tex

main.tex

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{arrows}
\usepackage[margin=0.6in]{geometry}

% Define a few styles for the nodes
\tikzset{
  tree_node/.style = {align=center, inner sep=0pt, text centered, font=\sffamily},
  square_node/.style = {tree_node, rectangle, draw=black, minimum width=1.5em, minimum height=1.5em, very thick},
  circle_node/.style = {tree_node, circle, black, draw=black, text width=1.5em, very thick},  
}

\begin{document}

\input{game-tree}

\end{document}