Exact solution methods
Linear programs (LP): Simplex algorithm
Integer programs (IP): branch and bound, branch and cut,
branch and price...
Mixed integer programs (MIP): branch and bound, branch and
cut, branch and price...
For many problems that can be formulated as IPs or MIPs, so far,
no polynomial time algorithms are known. This is true for most
vehicle routing problems.
For some problems the integrity requirements are fulfilled
automatically (Transportation Problem, Assignment Problem).
In some cases, the optimization problem can be formulated as an
IP or MIP but there exist algorithms that solve them in polynomial
time