Display count of leaves(depth) at each level in tree

Question

I am drawing a tree like given below. The MWE for it is below:

\documentclass[]{article}   
\usepackage{forest}   
\title{}   
\author{}   
\begin{document}   
\date{}   
\begin{forest}   
    [n   
    [n-1   
    [n-2[$\vdots$]][n-2[$\vdots$]]]   
    [n-1   
    [n-2[$\vdots$]][n-2[$\vdots$]]]]   
\end{forest}   
\end{document}  

I wish to write the count of number of nodes at each level of the tree. For example, adjacent to n, I wish to write 1. In the next level, 2 and so on. Since I am using forest package,

[n\hspace{1cm}1]

doesn't seem to help. Any simple ideas on how to do this?

Edit 1:

I found a related question which is not working for me.

Answer 1

Forest already counts levels, so a separate function to do this is superfluous. In addition, minimising the use of pgfmath will speed compilation. (Not an issue if this is your only tree and it is this simple, but more of a problem if you have many or complex trees.)

\documentclass[tikz,border=10pt]{standalone}
\usepackage[]{forest}
\begin{document}

\begin{forest}
  for tree=math content,
  before drawing tree={
    tikz+={\coordinate (a) at (current bounding box.east);},
    for nodewalk={fake=r, L, ancestors}{
      if={>O+t_+t={content}{\vdots}}{
        tikz+={\node [anchor=base west] at (.base -| a) {$2^k$};}
      }{%
        tikz+/.process={Ow+Pw}{level}{int(2^#1)}{\node [anchor=base west] at (.base -| a) {#1};}
      }
    }
  }
  [n   
    [n-1   
      [n-2
        [\vdots]
      ]
      [n-2
        [\vdots]
      ]
    ]   
    [n-1   
      [n-2
        [\vdots]
      ]
      [n-2
        [\vdots]
      ]
    ]
  ]   
\end{forest}   

\end{document}

Answer 2

A simple robust way would be to name one of the nodes in each level and use the y coordinate of that node (making use of the calc tikzlibrary) to get the correct height for each label. Counting the number of nodes can be done progrmatically - in your case the formula is clearly 2^n (use pgfs pow):

\documentclass[]{article}
\usepackage{forest}
\usetikzlibrary{calc} % let
\newcommand\leftsep{3} % How far left of center line of forest labels appear (cm)
\newcounter{levelcount} % Stores current level in tree
\newcommand\countnodes[1]{% Command to add label at height of node #1
    \draw let \p{L} = (#1) in (-\leftsep,\y{L}) node {\pgfmathparse{int(pow(2,\value{levelcount}))}\pgfmathresult};
    \stepcounter{levelcount} % Step counter for next level
}
\begin{document}
\begin{forest}
[n, name=root % Name root
[n-1, name=level1 % Name a node in level 1
[n-2, name=level2 [$\vdots$]][n-2[$\vdots$]]] % Name a node in level 2
[n-1
[n-2[$\vdots$]][n-2[$\vdots$]]]]
{% Node counting - these must be in order
\countnodes{root}
\countnodes{level1}
\countnodes{level2}
}
\end{forest}
\end{document}

Output: