Before revision: The general problem in topology optimization is correctly formulated as a double-\min mixed integer nonlinear programming (MINLP) problembased on the minimum total potential energy principle. It is proved that for linear elastic structures, the alternative iteration leads to a Knapsack problem, which is considered to be NP-hard in  computer science.
After revision: The general problem in topology optimization is correctly formulated as a bi-level knapsack problem, which is considered to be NP-hard in computer science.