Given a linear system $\dot{x} = Ax$, where $A$ is an $n \times n$ matrix
with $m$ nonzero entries, we consider the problem of finding the smallest set
of state variables to affect with an input so that the resulting system is
structurally controllable. We further assume we are given a set of "forbidden
state variables" $F$ which cannot be affected with an input and which we have
to avoid in our selection. Our main result is that this problem can be solved
deterministically in $O(n+m \sqrt{n})$ operations