\documentclass[12pt]{standalone}

\usepackage{tikz}

\begin{document}
\begin{tikzpicture}[every node/.style={circle,draw}]

\node at (0,0) {$1$}
    child {node {$2$}
        child {node {$3$}
            child {node {$4$}}
            child {node {$5$}}}};

\node at (4,0) {$1$}
    child {node {$2$}
        child {node {$3$}
            child {node {$5$}}}
        child {node {$4$}
            child {node {$6$}}}};

\end{tikzpicture}
\end{document}
