Check if a string is a subsequence of another string

In this problem, we will see how we can check whether a string is a subsequence of another string. Now we will consider an example to understand the given problem.

Let's say we have a string 'Welcome to OpenGenus' and a substring OGenus.
Now we need to find whether we have a substring or not.

mainString = S1 = "Welcome to OpenGenus";Subsequences (S2) are:OpenGenusOGenusWlcomepen [Notice we have deleted few characters in mainString]

Hence, S2 is a subsequence of S1 if we can convert S1 to ...