\documentclass[12pt]{standalone}

\usepackage{tikz}
\usepackage{ctex}

\begin{document}
\begin{tikzpicture}
    [level distance=1cm,
    level 1/.style={sibling distance=4cm},
    level 2/.style={sibling distance=2cm},
    level 3/.style={sibling distance=1cm}]
    \scoped[every node/.style={circle,draw,minimum size=8mm}]
    \node at (0,0) {1}
        child {node {2}
            child {node {4}}
            child {node {5}
                child {node {8}}
                child {node {9}}}}
        child {node {3}
            child {node {6}}
            child {node {7}
                child {node {10}}
                child {node {11}}}};

    \node at (0,-4cm) {完整二叉树（proper binary tree）};
\end{tikzpicture}
\end{document}
