I honestly don't think that you could gain and lose abs overnight but.
This is the workout you are going to have to do every other day if you want to get abs quickly.
Bicycle  Seated, pedal your feet like a bicycle.
In & out  Seated, bring your legs to your chest and back out.
Oblique V-up  Lay on your side, put your hand behind your head and bring your legs and head together. 25 per side.
Scissor  Lay on your back and move your legs in a scissoring motion, hold for 3 seconds per rep.
Leg raise  Lay on your back and bring both legs up straight.
Kayak  seated, extend your legs up 4-6 inches, clasp your hands and rotate your torso touching your fists to each side of your body.
Extra: You can replace this move with another 25 rep move.
pull up bar crunch  hang from a pull up bar and crunch your legs in to your chest.
This workout along with a diet consisting of high protein (eggs, fish, pork, meat, etc.) which will help you build the actual muscle. Fiber causing you to poop more often losing weight. And lastly, water to keep your muscles hydrated, of course!