input: Background: Computational complexity theory is a branch of the theory of computation in theoretical computer science that focuses on classifying computational problems according to their inherent difficulty, and relating those classes to each other. A computational problem is understood to be a task that is in principle amenable to being solved by a computer, which is equivalent to stating that the problem may be solved by mechanical application of mathematical steps, such as an algorithm.
  Question: What branch of theoretical computer science deals with broadly classifying computational problems by difficulty and class of relationship?
  Answer:<extra_id_0>.
output: <extra_id_0> Computational complexity theory<extra_id_1>