Implementing Generator in C++ (Generator, yield in Python)

This started from a LeetCode problem: 1286. Iterator for Combination. The most perfect implementation would use a Generator, namely Python’s yield. But I did not know how to do that, so I only implemented a solution that precomputes everything and stores the results. It was not elegant. After the contest, I learned an implementation of Generator in C++ and am sharing it here. I did not find a good Chinese article about this online.