\begin{definition}[Definition:Pre-Order Traversal of Labeled Tree]
Let $T$ be a binary labeled tree.
'''Pre-order traversal''' of $T$ is an algorithm designed to obtain a string representation of $T$.
The steps are as follows:
$\mathtt{Preorder} (T):$
* $n \gets t$, where $t$ is the root node of $T$.
* Output the label of $n$.
* If $n$ is a leaf node, stop.
* Let $T_1$ and $T_2$ be the left and right subtrees of $T$.
* If $n$ has only one child, skip this step. Output $\mathtt{Preorder} (T_1)$.
* Output $\mathtt{Preorder} (T_2)$.
* Stop.
The resulting string will be in Polish notation.
\end{definition}