Describe the hierarchy of computational complexity classes

The hierarchy of computational complexity classes organizes problems based on their computational difficulty and the resources required to solve them:

1. **P**: Problems solvable in polynomial time by a deterministic Turing machine.

2. **NP**: Problems for which a proposed solution can be verified in polynomial time by a deterministic Turing machine.

3. **NP-complete**: NP problems that are at least as hard as any problem in NP; if any NP-complete problem can be solved in polynomial time, then all NP problems can be.

4. **NP-hard**: Problems at least as hard as NP-complete problems but not necessarily in NP.

5. **PSPACE**: Problems solvable in polynomial space, which may require more time than NP problems.

This hierarchy reflects increasing levels of computational difficulty and resource requirements.