How can I add the line above this tree?

Question

I want to add a line 1st toss, 2nd toss, 3nd toss and outcome like this picture

I tried

\documentclass{standalone}
\usepackage{tikz}
\usepackage{tikz-qtree}
\begin{document}
\begin{tikzpicture}
\tikzset{grow'=right}
\tikzset{execute at begin node=\strut}
\tikzset{every tree node/.style={anchor=base west}}
\Tree [ [.H [.H [.H ][.T ]][.T [.H ][.T ]] ] [.T [. H [.H ][.T ] ][.T [.H ] [.T ]] ]] 
\end{tikzpicture}
\end{document}

and I got

Answer 1

Here's a forest solution which adds the labels automatically, as well as the contents of the nodes and the list of outcomes.

\documentclass[tikz,border=10pt]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
  /tikz/every node/.append style={font=\sffamily},
  toss/.style={
    before drawing tree={
      tikz/.wrap pgfmath arg={\node at ([yshift=10pt].center |- h.north) {Toss ##1};}{level()},
    }
  },
  tosses/.style={
    name=h,
    for ancestors'={if level=0{}{toss}},
    before packing={
      !1.tikz={\node at ([yshift=10pt].center |- h.north) {Outcomes};}
    }
  },
  for tree={
    grow'=0,
    parent anchor=east,
    child anchor=west,
    anchor=west,
    tier/.wrap pgfmath arg={tier #1}{level()},
    l sep+=20pt,
    font=\sffamily,
    delay={
      if level=0{}{
        if n=1{content=H}{content=T}
      }
    }
  },
  before typesetting nodes={
    where n children=0{
      append={[, edge={dashed}, anchor=west, font=\sffamily, content/.wrap 3 pgfmath args={(#1,#2,#3)}{content("!uuu")}{content("!uu")}{content("!u")}]}
    }{}
  }
  [
    [
      [
        [, tosses]
        []
      ]
      [
        []
        []
      ]
    ]
    [
      [
        []
        []
      ]
      [
        []
        []
      ]
    ]
  ]
\end{forest}
\end{document}

A brief introduction to forest can be found in the second part of my answer here.

The complexity of the configuration here is only due to the automatisation, which relies on doing particular things at particular stages of the drawing process. The actual tree specification is simple.

[Indeed it is just a bunch of square brackets, one comma and the string tosses. What could be simpler than that?!]

EDIT

You can adjust the contents of the nodes and labels in the preamble of the tree. I've added three comments to highlight the points at which this should be done and substituted a circle and filled circle from pifont for H and T to illustrate the idea. Since this presumably makes 'Toss' inappropriate, I've also changed the label prefix to 'Ball'.

\documentclass[tikz,border=10pt]{standalone}
\usepackage{forest,pifont}
\begin{document}
\begin{forest}
  /tikz/every node/.append style={font=\sffamily},
  toss/.style={
    before drawing tree={% specify the prefix of the labels at the top here
      tikz/.wrap pgfmath arg={\node at ([yshift=10pt].center |- h.north) {Ball ##1};}{level()},
    }
  },
  tosses/.style={
    name=h,
    for ancestors'={if level=0{}{toss}},
    before packing={% specify the label for the final (rightmost) level here
      !1.tikz={\node at ([yshift=10pt].center |- h.north) {Outcomes};}
    }
  },
  for tree={
    grow'=0,
    parent anchor=east,
    child anchor=west,
    anchor=west,
    tier/.wrap pgfmath arg={tier #1}{level()},
    l sep+=20pt,
    font=\sffamily,
    delay={
      if level=0{}{% specify the contents of the nodes here
        if n=1{content=\ding{109}}{content=\ding{108}},
      }
    }
  },
  before typesetting nodes={
    where n children=0{
      append={[, edge={dashed}, anchor=west, font=\sffamily, content/.wrap 3 pgfmath args={(#1,#2,#3)}{content("!uuu")}{content("!uu")}{content("!u")}]}
    }{}
  }
  [
    [
      [
        [, tosses]
        []
      ]
      [
        []
        []
      ]
    ]
    [
      [
        []
        []
      ]
      [
        []
        []
      ]
    ]
  ]
\end{forest}
\end{document}

Answer 2

The easiest way given what you've done is to draw two trees, one on top of the other using TikZ's scope mechanism. I've increased the level distance of the tree branches except the last branch to accommodate the labels, and manually shifted the label tree to the left to line up the labels better.

\documentclass{article}
\usepackage{tikz}
\usepackage{tikz-qtree}
\begin{document}
\begin{tikzpicture}
\tikzset{grow'=right}
\tikzset{execute at begin node=\strut}
\tikzset{every tree node/.style={anchor=base west}, level distance=1in,level 4+/.style={level distance=.5in,edge from parent/.append style={dashed}}}
\Tree [ 
    [.H 
        [.H 
            [.H [.{(H,H,H)} ]]
            [.T [.{(H,H,T)} ]]]
        [.T 
            [.H [.{(H,T,H)} ]]
            [.T [.{(H,T,T)} ]]]] 
    [.T 
        [.H 
            [.H [.{(T,H,H)} ]]
            [.T [.{(T,H,T)} ]]]
        [.T 
            [.H [.{(T,T,H)} ]] 
            [.T [.{(T,T,T)} ]]]]
    ]
\begin{scope}[xshift=-1cm,yshift=4.5cm, edge from parent/.style={draw=none},align=center,level 4+/.style={level distance=.8in}]
\Tree [.{} [.{First Toss} [.{Second Toss} [.{Third Toss} [.{Outcomes} ]  ] ] ] ]
\end{scope}
\end{tikzpicture}
\end{document}

For a similar question see: How can I align captions with each level of a tree drawn with tikz-qtree?