En este post, varios links sobre el problema de la mochila.
Entrada de Wikipedia: https://en.wikipedia.org/wiki/Knapsack_problem
Explicación en Stack Overflow: https://stackoverflow.com/questions/3907545/how-to-understand-the-knapsack-problem-is-np-complete.
Polynomial-time approximation scheme: https://en.wikipedia.org/wiki/Polynomial-timeapproximationscheme
Pseudo-polynomial time: https://en.wikipedia.org/wiki/Pseudo-polynomial_time