How to understand subsequence?

Question

C++ / C#

For example, there is a sequence of pairs. This is clear. What does subsequence mean? How is it implemented in C++?

Answer 1

This is any sequence consisting of a subset of the elements of the sequence that preserves their order.
As you come up with, so it will work. A vector or array of booleans can specify which elements of a sequence are selected to participate in a subsequence. Or a set of indexes of elements. Or an increasing array/vector of indices.