Let $f:\mathbb{C}\to\mathbb{C}$ be an entire function.
Let $n\in\mathbb{N}$ and suppose that
$$\forall w\in\mathbb{C}:\#\{z\in\mathbb{C}:f(z)=w\}\leq n$$
In words, every complex value is attained by $f$ in at most $n$ different places.
Prove that $f$ is a polynomial of degree at most $n$.
This follows from Picard's great theorem.