On the Classification of NP Complete Problems and their Duality Feature

NP Complete (abbreviated as NPC) problems, standing at the crux of deciding whether P=NP, are among hardest problems in computer science and other related areas. Through decades, NPC problems are treated as one class. Observing that NPC problems have different natures, it is unlikely that they will have the same complexity. Our intensive study shows that NPC problems are not all equivalent in computational complexity, and they can be further classified. We then show that the classification of NPC problems may depend on their natures, reduction methods, exact algorithms, and the boundary between P and NP. And a new perspective is provided: both P problems and NPC problems have the duality feature in terms of computational complexity of asymptotic efficiency of algorithms. We also discuss about the NPC problems in real-life and shine some lights on finding better solutions to NPC problems.