Drawing a tree where many nodes have the same child in TikZ

Question

I'm trying the draw a tree using the TikZ package, but I don't know how to draw a child node that has many parents. An example for that, is given by this picture

My problem is how to draw the level 3.

Answer 1

You can use two trees sitting on top of each other.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[sibling distance=2.5cm]
\node (topnode) at (0,5) { {\{\{1\}\{2\}\}\{3\}\{4\}}\} } 
child { node {{\{\{1,2\}\{3\}\{4\}}\}} }
child { node {{\{\{1,3\}\{2\}\{4\}}\}} }
child { node {{\{\{1,4\}\{2\}\{3\}}\}} }
child { node {{\{\{2,3\}\{1\}\{4\}}\}} }
child { node {{\{\{2,4\}\{1\}\{3\}}\}} }
child { node {{\{\{3,4\}\{1\}\{2\}}\}} }
;

\node[minimum width=3cm](bottomnode) {\{\{1,2,3,4\}\}} [grow'=up]
child { node {{\{\{1,2,3\}\{4\}}\}} }
child { node {{\{\{1,2,4\}\{3\}}\}} }
child { node {{\{\{1,2\}\{3,4\}}\}} }
child { node {{\{\{1,3,4\}\{2\}}\}} }
child { node {{\{\{1,3\}\{2,4\}}\}} }
child { node {{\{\{1,4\}\{2,3\}}\}} }
child { node {{\{\{2,3,4\}\{1\}}\}} }
;
\foreach \x in {1,2,3}{
\draw (topnode-1) -- (bottomnode-\x);
}
\end{tikzpicture}
\end{document}

I'll leave the connection lines to you if you don't mind. You can access each node by first writing up the parent node name followed by a dash and the item number e.g. (topnode-5).