Let me list a few areas and why you should study them:
Algebra / Number Theory . There are many theoretical numerical algorithms, and at some point you may need to use / modify the existing one. Knowledge of the theory underlying their work greatly facilitates their memorization (and restores them if you do not forget;)). A classic of such an algorithm is the Euclidean algorithm for finding the greatest common divisor of two numbers. More complex algorithms, such as quadratic sieves, etc., are widely used in cryptography.
Graph theory . I hope I do not need to explain it too much. Charts are used to solve many many problems in computer science. They are used to represent networks, relations between objects, as well as a huge class of special applications. Algorithms include the shortest path, network stream, coloring, etc.
Linear Algebra : Again, there are many scientific applications of linear algebra. I think I can say with confidence that the main use of linear algebra is to solve systems of equations. Complex differential equations are solved using linear algebra. Knowing how to multiply huge matrices and solve huge systems of linear equations lies at the heart of many computer simulations. More closely related to everyday computing, computer graphics (especially 3D graphics) are based on linear algebra.
Calculus . It should also be self-evident. You will not be very far to do anything related to mathematics and science, without knowing the calculus. While the curse of college students, it really is everywhere in modern science.
Probability . There are a number of probabilistic algorithms that rely on the likelihood of doing their dirty work. Also, modeling can also use randomness and the likelihood of good results. Itโs not very difficult to learn the basics of probability that you really need.
Statistics . If you are writing a program that calculates statistics and performs statistical tests, or you yourself use statistics, this is a very useful tool for decoding and interpreting information. Indeed, anyone, not just mathematicians and scientists, should have at least the basic characteristics of statistics.
Numerical analysis . This is a kind of trick. This is what connects mathematics and computer science. Math works well and well on paper, but when you start discretizing and limiting real numbers, you will have problems. This is what numerical analysis is for. It describes the various numerical problems that you may encounter when working with these mathematical algorithms, as well as math / computer tricks that you can use to avoid / fix them.