how to reduce the distance between the picture caption and the picture?

Question

I found that the distance between the picture caption and the picture is a little large, as we can see in the picture

I use the code as follows, thus, how to reduce the distance between the caption and the picture?

\usepackage[latin1]{inputenc}
\usetikzlibrary{trees}
\usepackage{verbatim}
\usetikzlibrary{trees,decorations.pathmorphing}
 \begin{figure}[ht!]
{\centering
\tikzset{level 1/.style={level distance=1.0cm, sibling distance=0.5cm}}
\tikzset{bag/.style={text width=20em, text centered,yshift=-0.2cm}}
\begin{tikzpicture}[grow=down, -stealth,  edge from parent/.style={draw,decorate,decoration={snake, post=lineto, post length=2mm}}]
    {\node[bag]{$S_0{:}(\{a{\mapsto}1\},A{<}a{=}1{>}B,t_{i_1}{\geq}0,0)$}
         child{edge from parent node{$$}; \node[bag]{$S_{n-1}{:}(...,stop_{good},t_{i_3}{\geq}3,0)$}
    };
    }
\end{tikzpicture}
}
\caption{LTS of A Pick Activity}
\end{figure}

Answer 1

Alternatively, you can use the skip key of the caption package; I used a value of 2pt, but you can choose the length that best suits your needs:

\documentclass{article}
\usepackage[skip=2pt]{caption}
\usepackage{tikz}
\usetikzlibrary{trees}

\begin{document}

\usetikzlibrary{trees,decorations.pathmorphing}
 \begin{figure}[ht!]
{\centering
\tikzset{level 1/.style={level distance=1.0cm, sibling distance=0.5cm}}
\tikzset{bag/.style={text width=20em, text centered,yshift=-0.2cm}}
\begin{tikzpicture}[grow=down, -stealth,  edge from parent/.style={draw,decorate,decoration={snake, post=lineto, post length=2mm}}]
    {\node[bag]{$S_0{:}(\{a{\mapsto}1\},A{<}a{=}1{>}B,t_{i_1}{\geq}0,0)$}
         child{edge from parent node{$$}; \node[bag]{$S_{n-1}{:}(...,stop_{good},t_{i_3}{\geq}3,0)$}
    };
    }
\end{tikzpicture}
}
\caption{LTS of A Pick Activity}
\end{figure}

\end{document}

Answer 2

The spaces before and after a caption are controlled by \abovecaptionskip and \belowcaptionskip. Thus you can change the distance before the caption with

\setlength{\abovecaptionskip}{0cm}

Change the value as appropriate.