Explain the P vs NP problem and why it is considered one of the most important open problems in computer science. Clearly define classes P and NP, providing concrete examples of problems in each class. Discuss the concept of NP-completeness and provide at least two examples of NP-complete problems. Explain the implications if P were equal to NP versus if they were distinct, touching on practical consequences for cryptography, optimization, and problem-solving in general.