Swampscott cops are investigating after a 23-year-old man standing in his living room was struck by a bullet that flew into his house, police said.Officers responding to the 4 a.m. shooting on Erie Street found the man injured in his home, according to police.Investigators later determined the man, whose name was not released last night, was shot in the arm by someone outside the house.The victim was rushed by ambulance to Salem Hospital with non-life-threatening injuries that required surgery, police said.