\begin{tikzpicture}[ level 1/.style={sibling distance=2.2cm, level distance=0.8cm}, % 第1层子节点之间水平间距2.6cm，第一层和第二层之间高度1cm level 2/.style={sibling distance=1.2cm, level distance=0.8cm}, % 第2层子节点间距 level 3/.style={sibling distance=1.2cm, level distance=0.8cm}, line width=0.7pt, % 普通节点样式（带圆圈） node style/.style={ circle, draw, minimum size=1.7em, line width=0.7pt, inner sep=2.5pt }, % NULL节点样式（无圆圈） null node/.style={minimum size=1.7em} ] % 根节点A（普通样式） \node[node style] (A) {\ttfamily A} % 子节点B（普通样式） child {node[node style] (B) {\ttfamily B} child {node[null node] {\ttfamily NULL}} % NULL无圆圈 child {node[null node] {\ttfamily NULL}} } child {node[node style] (C) {\ttfamily C} child {node[null node] {\ttfamily NULL}} child {node[node style] (D) {\ttfamily D} child {node[null node] {\ttfamily NULL}} child {node[null node] {\ttfamily NULL}} } }; \end{tikzpicture}代码运行生成的图如下：
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