# Problem with binary tree

I am drawing AVL trees using tikz. I have this code:

\documentclass{scrartcl}
\usepackage{tikz}
\usetikzlibrary{trees}

\begin{document}
\begin{center}
\begin{tikzpicture}
\node[circle,draw](z){19}
child{node[circle,draw]{16}
child{node[circle,draw]{11}}
child{node[circle,draw]{17}}}
child{node[circle,draw]{21}
child{node[circle,draw]{20}}
child{node[circle,draw]{26}}};
\end{tikzpicture}
\end{center}
\end{document}


It should be a balanced AVL tree. The right child of the 16 should be 17 and the left child of the 21 should be 20. With the code I get this

As you can see the 17 and the 20 are fused as one node. How can I solve this problem without modifying all other trees that I have drawn already?

You can specify a sibling distance for each level.

\documentclass{scrartcl}
\usepackage{tikz}
\usetikzlibrary{trees}

\begin{document}
\begin{center}
\begin{tikzpicture}[
every node/.style={circle, draw},
level 1/.style={sibling distance=6em},
level 2/.style={sibling distance=3em},
]
\node (z){19}
child{node {16}
child{node {11}}
child{node {17}}}
child{node {21}
child{node {20}}
child{node {26}}};
\end{tikzpicture}
\end{center}
\end{document}


I have also added a style for every node, which cleans up your code.

With use of the forest package the tree code is simpler and nodes never overlap:

\documentclass{scrartcl}
\usepackage{forest}

\begin{document}
\begin{center}
\begin{forest}
for tree = {
% nodes
circle, draw,
minimum size = 2em,
inner sep = 2pt,
% tree
grow = south,
l sep = 9mm,  % vertical distances between nodes
s sep = 3mm   % horizontal distances between nodes
}
[19
[16
[11]
[17]
]
[21
[20]
[26]
];
]
\end{forest}
\end{center}
\end{document}