Brilliance in Mystery Novels

Am reading The Mysterious Affair at Styles. It's Agatha Christie's first book, written in 1920. This is very early for mysteries and the debt to Sherlock Holmes is evident.

I've always wondered how authors write mysteries. It must be close to computer programming. Working out the crime, the suspects movements, and then the order in which clues are discovered so as to hint but not give away the murderer. Then I came across this. Wikipedia has this "Agatha Christie told him that she wrote her books up to the last chapter, and then decided who the most unlikely suspect was. She would then go back and make the necessary changes to 'frame' that person."

What a brilliantly unorthodox approach! It's maximizes surprise because not even the author knows the murder while the book is being written. If the story has enough suspects and red herrings then going back and "framing" that person should not be too hard.

Another case of a brilliant idea being a simple one that tackles complexity by simply avoiding it in the first place.