CHATHAM, Mass. — From drones and smartphone apps to old-school flags and signs, a growing great white shark population along the East Coast has officials and researchers turning to responses both high- and low-tech to ensure safety for millions of beachgoers this summer. On Cape Cod, Massachusetts, new warning flags and signs are cropping up at some of the coastline's most popular beaches and a local shark research nonprofit is developing a shark tracking app for the entire Eastern Seaboard. Researchers at Duke University and the University of North Carolina, meanwhile, are testing shark-seeking drones in a scientific study that may one day give beach lifeguards a new eye in the sky. "The days of burying our heads in the sand and saying, 'What sharks?