Throughout my first two years taking computer science and math courses, there’ve been times when an instructors recommends using \(\LaTeX\). That being said, a lot of students groan at the very words. Being the ambitious freshman that I was last year, I tackled it head on. And I have to say that latex does make your homework look a lot nicer and exactly how the textbooks typeset graphics and equations. Two of the most common graphics that you’ll have to create in computer science classes are graphs and trees without a doubt. And today, I’ll be showing you how to draw trees.

There are two packages that I recommend to draw trees:

qtree
tikz

qtree

The qtree package is probably the fastest way to draw trees in latex. If you’re ever in the situation where your homework is due in less than a day, this is the package to use. The syntax is simple, a two level binary tree drawn by qtree looks like this:

\Tree [.A B C ]

In this code, the . signifies the parent node. And B and C are the children. Note the space between the last child, C, and the ending bracket. As with most algorithms involving trees, to create subtrees, do this recursively. This means to have a left subtree with B as the parent, the code looks like:

\Tree [.A [.B D E ] C ] % here we substituted B with [.B D E ]

As you can see, it requires very little code to draw it. Compared to students who spend time drawing this up on Google Drawings, you can produce and edit your trees much faster.

tikz

The tikz package it probably what most people use to draw graphs and trees, and with good reason. It allows you a lot of customization that the qtree doesn’t offer. Syntax for drawing trees with tikz is just as simple, although not as quick. To draw the same the tree with tikz the code is:

\begin{tikzpicture}
\node (A){A}
  child{ node (B){B} }
  child{ node (C){C} };
\end{tikzpicture}

Notice the semicolon, after the last defined child of A, you need that or your code won’t compile. The text in parentheses (B) signifies the name of the node that you can refer to when doing relative positioning. And the text in the braces {B} following the node’s name, is the text that will be displayed as the node in the tree.

To draw the same subtree in the qtree example:

\begin{tikzpicture}
\node (A){A}
  child{
    % we replace \Node(B){B} with:
    node (B){B}
    child { node (D){D} }
    child { node (E){E} }
  }
  child{ node (C){C} };
\end{tikzpicture}

tikz caveats

Relative positioning

Before I mentioned relative positioning, so instead of specifying coordinates to place nodes:

\node (A) at (1,1) {A};
\node (B) at (1,2) {B};

you can do something like:

\node (A) {A};
\node (B) [right =of A] {B};

To do this you need to use tell tikz to use the positioning package.

\usetikzlibrary{poisitioning}

Put this before begining a tikzpicture. The positioning words that are usable are:

above
below
right
left

And you can mix and match the vertical and horizontal positioning words as your heart desires. So [below right =of A] works just fine.

Every node style

You can set the style for every node in one line, so you don’t need to specifiy the style redundantly. This can be done by something like:

\tikzstyle{every node} = [circle, minimum size=18pt, draw]

This example tells latex to draw circles around each node and have the text at 18pt font.

Tree edge labels

For sure if you’re drawing trees, you’ll want to draw a trie at some point. To label the edges in the two level binary tree in the previous examples:

\node (A){A}
  child{ 
    node(B){B}
    edge from parent node[left, draw=none]{0} % labels the edge (A, B) with 0
  }
  child{ node(C){C} };

The first argument left in the brackets tells which side to put the label, and draw=none tells latex not to draw the circle if you’ve previously defined a style for every node. Because essentially this label is a node.

Node distance

There will come a time when you have a lot of children for one node and they will overlap each other. To fix this, we can set node distance for the tikzpicture setting. This means doing something like \begin{tikzpicture}[node distance = 3cm] to space the nodes apart. The distance is measured from the center of each node to the other. So keep that in mind when setting the distance.