I'm reading the **Introduction to Algorithms** book and there's the following recurrence relation

$$
 T(n) = \begin{cases} c, & n = 1 \\ 2T(n/2) + cn, & n > 1 \end{cases}
$$

$c$ is just a constant.

So, the author wants to show that the solution is $\mathcal{O}(nlgn)$ and for this purpose he draws a tree.
[![enter image description here](https://i.stack.imgur.com/qLue2.png)](https://i.stack.imgur.com/qLue2.png)

Could someone please explain me why the author considers $cn$ as the root of the tree? I expected that the root is $T(n)$ and it has two subtrees and one leaf ($cn$).