Approximation
Polynomial-time approximation scheme
In computer science, a polynomial-time approximation scheme (PTAS) is a type of approximation algorithm for optimization problems (most often, NP-hard optimization problems).
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NP, P, NP-complete and NP-Hard
To solve the problem of NP-completeness. Firstly, we need to figure out what is NP, P, NP-complete and NP-Hard.