Number theory. Number theory is closely related to high-end science. Mastering number theory will benefit you for life. Game theory focuses on the field of economics, but it is not perfect and abstract, and its position in the mainstream theory of economics cannot be shaken. Our country focuses on institutional economics, but game theory is still quite important. The last point is statistics. Statistical mechanics is closely related to quantum mechanics in physics. If a quantum physicist needs you in the future, maybe you can help him win the Nobel Prize, which is quite possible. But statistics may be the most difficult for you, because you must master it. It is used in economics as well as probability theory and game theory. China needs such elites in economics and mathematics, and science needs such talents in mathematics.