I don't know what all the fuss is over youth in Asia. (Benny Hill reference, sorry)
Many states use the single injection method used to put down pets and other animals, but it does take considerable longer for death to occur. It's essentially like taking an overdose of sleeping pills, you relax, go to sleep, and as the sleep gets deeper your diaphragm slowly stops working so you stop breathing.
Some people think "cruel and unusual" means the death has to be painless. I think cruel and unusual would be any unnecessary, prolonged pain intentionally induced for revenge. I've always thought the electric chair was that very thing, a very violent death that affects every nerve in the body in a painful way. The single injection method seems like the best overall solution.