答案1
这就是你所想的吗?
代码在这里:
\documentclass{article}
\begin{document}
\[
\begin{array}{rc@{}c@{}c@{}c}
\mbox{Used data structures}&=&\underbrace{S}&\underbrace{S'}&\underbrace{T}\\
&&\downarrow&\downarrow&\downarrow\\
&&\begin{tabular}{c}\scriptsize{Explicit}\\[-.75ex] \scriptsize{Stack}\end{tabular}&
\begin{tabular}{c}\scriptsize{Implicit}\\[-.75ex] \scriptsize{Stack}\end{tabular}&
\begin{tabular}{c}\scriptsize{Trailer}\\[-.75ex] \scriptsize{Stack}\end{tabular}
\end{array}
\]
\end{document}
答案2
\documentclass{article}
\usepackage{amsmath}
\def\Stack#1#2{\underbrace{#1}_{\underset{%
\begin{subarray}{c}
\text{\vphantom{p}#2}\\\text{Stack}
\end{subarray}}
{\downarrow}}}
\begin{document}
\begin{equation*}
\text{Used data structures}=\Stack{S}{Explicit}\ \Stack{S'}{Implicit}\ \Stack{T}{Trailer}
\end{equation*}
\end{document}