How do I make an unbalanced binary tree?

Question

How do I draw just a tree with one child using tikzpicture?

As shown in the picture, I want to have a tree like (1).

But I have only been succesful in getting a picture like (2) and (3) using:

\begin{tikzpicture}
\node[circle,draw](z){$30$}
% comment the below for (3):
child{}
child{
node[circle,draw]{40}}
;
\end{tikzpicture}

So, is there a modifier that I don't know of? I've tried to find it in the manual of 7xx pages, but I failed on finding something useful.

Answer 1

You can use missing children (Section 18.5.3 Missing Children in the pgf manual):

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\node[circle,draw](z){$30$}
  child[missing]{}
  child{
    node[circle,draw]{40} child{node[circle,draw] {20}} child[missing] };
\end{tikzpicture}

\end{document}

Answer 2

You can also do this with tikz-qtree, which has a simpler syntax, especially for large trees and if most of your nodes are balanced, and only a few aren't.

\documentclass{article}
\usepackage{tikz-qtree}
\begin{document}
\tikzset{every tree node/.style={minimum width=2em,draw,circle},
         blank/.style={draw=none},
         edge from parent/.style=
         {draw, edge from parent path={(\tikzparentnode) -- (\tikzchildnode)}},
         level distance=1.5cm}
\begin{tikzpicture}
\Tree
[.50     
    [.60 ]
    [.30  
    \edge[blank]; \node[blank]{};
    \edge[]; [.40
             \edge[]; {20}
             \edge[blank]; \node[blank]{};
         ]
    ]
]
\end{tikzpicture}
\end{document}