How to draw a graph with ring in latex?

Question

As the title tells, how to draw a graph with ring in latex? I have learned that I can use forest to draw a tree in latex. For example, using following code, I can have one simple binary tree.1->2, 1->3.

\begin{forest}
for tree={circle,draw}
[1[2][3]]
\end{forest}

Then, Im wondering how to draw a graph with ring. For example, how can I add another edge between 1 and 2 so that there is ring between node 1 and 2. 1 -> 2, and 2->1. Also, I'm wondering how to draw a self ring. i.e., 1->1(an egde starting from node 1 and ending at node 1)

Answer 1

Forest is designed to draw trees. If what you need is mostly a tree, you can still use it with a bit of manual intervention. If you really want to draw a generic graph, though, it is better to use something designed for that.

In case you want basically-a-tree-with-the-odd-non-tree-bit, here's one solution.

\documentclass[border=10pt,multi,tikz]{standalone}
\usepackage{forest}
\begin{document}
\begin{forest}
  for tree={%
    circle,
    draw,
    edge={->},
  }
  [1, tikz+={\draw () edge [loop above] ();}
    [2, edge path'={(!u.parent anchor) edge [bend right, <-]  (.north) (!u.parent anchor) edge [bend left] (.north east)}]
    [3]
  ]
\end{forest}
\end{document}

If you need to draw generic graphs, TikZ offers built-in graph-drawing facilities, courtesy of LuaTeX. Here's one using the binary tree layout, which is not terribly fussy about whether the specification is technically a tree or not, as the following shows. Note that this code must be compiled with LuaTeX - it will not work with other engines.

\RequirePackage{luatex85}
\documentclass[border=10pt,multi,tikz]{standalone}
\usetikzlibrary{graphs,graphdrawing}
\usegdlibrary{trees}
\begin{document}
\begin{tikzpicture}
  \graph
  [
    binary tree layout,
    nodes={draw, circle},
  ]
  {
    1 ->[bend right] 2 -> [bend right] 1 ->[loop above] 1 -> 3;
  }
  ;
\end{tikzpicture}
\end{document}