beamer中的箭头

\documentclass{beamer}
\usepackage{tikz}
\usepackage{amsmath}
\usetikzlibrary{arrows,shapes}

\author{Author}
\title{Presentation title}

\begin{document}

\tikzstyle{every picture}+=[remember picture] %a global declaration
\begin{frame}

\tikzstyle{na} = [baseline=-.5ex] % 箭头要下移的距离,后面\tikz[na]要用到的。

\begin{itemize}[<+-| alert@+>]
\item A
\tikz[na] \node[coordinate] (n1) {}; %
\end{itemize}

% Below we mix an ordinary equation with TikZ nodes. Note that we have to
% adjust the baseline of the nodes to get proper alignment with the rest of
% the equation.
$
a =
\tikz[baseline]{
\node[fill=blue!20,anchor=base] (t1)
{$ 2\omega$};
} +
\tikz[baseline]{
\node[fill=red!20, ellipse,anchor=base] (t2)
{$\alpha\times r$};
} +
\tikz[baseline]{
\node[fill=green!20,anchor=base] (t3)
{$\omega\times r$};
}
$

\begin{itemize}[<+-| alert@+>]
\item B
\tikz[na]\node [coordinate] (n2) {};
\item C
\tikz[na]\node [coordinate] (n3) {};
\end{itemize}

% Now it's time to draw some edges between the global nodes. Note that we
% have to apply the 'overlay' style.
\begin{tikzpicture}[overlay]
\path[line width=3,->]<1-> (n1) edge [bend left] (t1); %[bend left] 箭头向箭头的左方凸起
\path[<->]<2-> (n2) edge [bend right] (t2);%[bend right] 箭头向箭头的右方凸起
\path[color=red,->>]<3-> (n3) edge [out=0, in=-90] (t3); %[out=0,in=-90] 箭头发起水平，结束是向上的90度角。
\end{tikzpicture}
\end{frame}
\end{document}