Before revision:  More specifically, our contribution is two-fold : 1. In the centralized setting, we provide a surprisingly simple combinatorial algorithm that is asymptotically optimal in terms of both approximation factor and running time : an O(\alpha)-approximation in linear time.
After revision:  More specifically, our contribution is two-fold : 1. In the centralized setting, we provide a surprisingly simple combinatorial algorithm that is asymptotically optimal in terms of both approximation factor and runtime : an O(\alpha)-approximation in linear time.