Crossed hierarchy

Question

I appreciate it if you let me know the most elegant way to draw a crossed hierarchy such as the followings:

    X
   /\
  Y  Z
  /\/\
 p q  t

q has two parents Y and Z.

Answer 1

This is a hack because it uses the fact that the distance between nodes doesn't change from level to level and so by default branches will overlap:

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node {X} 
   child {
     node {Y}
     child {
        node {p}
     }
     child {
        node {q}
     }
   }
   child {
      node {Z}      
      child {
         node {\phantom{q}}      
      }
      child {
         node {t}      
      }
   }
;
\end{tikzpicture}
\end{document}

The edge from Z to q is not really to the q but to an invisible q that sits on top of the first q.

A better way to do it is to name the nodes and draw the extra edge after the tree. This way if the q node moves the extra edge will still point to it. You can create a missing child node to keep the spacing as if there were an actual first child.

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[
   level 1/.style={sibling distance=20mm},
   level 2/.style={sibling distance=15mm}
]
\node {X} 
   child {
     node {Y}
     child {
        node {p}
     }
     child {
        node (q) {q}
     }
   }
   child {
      node (Z) {Z}      
      child[missing] {
         node {q}
      }
      child {
         node {t}      
      }
   }
;
\draw (Z) -- (q);
\end{tikzpicture}
\end{document}