[Tex/LaTex] How to control spacing in a TikZ tree

Question

I want to draw a tree and I don't know how to enforce the spacing between them. Some are overlapping.

This is my code:

\begin{tikzpicture}
\tikzstyle{every node}=[circle,draw]
\node {A}
child { node {B} 
child { node {E} }
child { node {F} child { node {K} } child { node {L} } child { node {M}}}
child { node {G} }}
child { node {C} 
child {node {H}}}
child { node {D} child {node {I}} child {node {J} child {node {N}} child {node {O}}}};
\end{tikzpicture}

The nodes G and H are overlapping.

I've tried using the parameter node distance but it doesn't work.

Answer 1

Refer to Section 18.4 Specifying Options for Trees and Children of the pgfmanual. Using sibling distance, you can solve the problem (use the values that best suit your needs):

\documentclass{article}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[every node/.style={circle,draw},level 1/.style={sibling distance=30mm},level 2/.style={sibling distance=10mm}
]
\node {A}
child { node {B} 
child { node {E} }
child { node {F} child { node {K} } child { node {L} } child { node {M}}}
child { node {G} }}
child { node {C} 
child {node {H}}}
child { node {D} child {node {I}} child {node {J} child {node {N}} child {node {O}}}}
;
\end{tikzpicture}

\end{document}