Before revision:  We provide a constructive nondeterministic algorithm to solve the cutpoint isolation problem, even for exponentially ambiguousPFA, and we also show that the problem is at least NP-hard  .
After revision:  We provide a constructive nondeterministic algorithm to solve the cutpoint isolation problem, which holds even when the PFA is exponentially ambiguous. We also show that the problem is at least NP-hard .