PL-OM 1992 Problem 2

Find all functions $f : \mathbb{Q}^{+} \rightarrow \mathbb{Q}^{+}$ , where $\mathbb{Q}^{+}$ is the set of positive rationals, such that $f(x+1) = f(x) + 1$ and $f(x^3) = f(x)^3$ for all $x$ .