\begin{figure}
\centering
\tikzstyle{Arrow} = [
thick,
decoration={
markings,
mark=at position 1 with {
\arrow[thick]{latex}
}
},
shorten >= 3pt, preaction = {decorate}
]
\begin{tikzpicture}[node distance = 1.4cm]
\node [circle, draw] (3) {$v_1$};
\node [circle, draw, right =of 3] (5) {$v_2$};
\node [circle, draw, right =of 5] (6) {$v_3$};
\node [circle, draw, left =of 3] (1) {$v_4$};
\node [circle, draw, below =of 3] (2) {$v_5$};
\node [circle, draw, above =of 5] (4) {$v_6$};
\draw[Arrow] (1.south) -- (2.west);
\draw[Arrow] (1.east) -- (3.west);
\draw[Arrow] (2.east) -- (5.south);
\draw[Arrow] (3.north) -- (4.west);
\draw[Arrow] (3.east) -- (5.west);
\draw[Arrow] (5.east) -- (6.west);
\end{tikzpicture}
\caption{示例}
\end{figure}
效果如下图: