Math for hackers.
4180122: Basic linear algebra is extremely important in conjuncture with rudimentary matrix manipulation.  I am always astounded by the number of college graduates who can't perform simple matrix multiplications, cross-products, dot-products, L-U factorization, etc without references.Without these basics, it becomes increasing difficult to grasp more advanced subjects such as (non)linear optimization, calculating computation complexity, numerical methods, root finding, game theory, and so on. The importance of calculus and combinatorics, as mentioned, should not be undermined either.EDIT: How could I have forgotten about Eigen Decompositions! I never fully understood its significance until the last two years of college when all fragmented subjects I had learned (in terms of CS, Engineering, and Control Theory) finally fell into place.