In computational complexity theory, is a class of problems informally "at least as hard as problems in NP." A problem H is NP-hard if and only if there is an NP-complete problem L that is polynomial time Turing-reducible to H. In other words, L can be solved in polynomial time by an oracle machine with an oracle for H. Informally we can think of an algorithm that can call such an oracle machine as subroutine for solving H, and solves L in polynomial time if the subroutine call takes only one step to compute. NP-hard problems may be of any type: decision problems, search problems, optimization problems.