A woman burned to death in the elevator of her Brooklyn apartment building Saturday after a man ambushed her, sprayed her with liquid and set her afire with a Molotov cocktail, police said. The unidentified man was waiting for the woman, who was in her 60s, when the elevator doors opened to her floor of the Prospect Heights building. The man sprayed her with an accelerant and set her on fire, New York City police spokesman Paul Browne said.