$2.$ Forgetful Vector $-$ C$++$

In the folder $$ForgetfulVector$$ is an implementation and header file. Write a class, named ForgetfulVector, that acts like a vector in that it supports the size, at$,$ and push$\_$back methods. This class only needs to support the storage of ints $($templated classes will be taught later$).$ The one strange thing is that the ForgetfulVector $$forgets$$ the first time it encounters a value, and doesn$$t present that value or give it an index.

Example use:

std::vector vec $=\{1,2,3,2\}$; ForgetfulVector fv$()$; fv$.$push$\_$back$(1)$; fv$.$push$\_$back$(2)$; fv$.$push$\_$back$(3)$; fv$.$push$\_$back$(2)$; $//$ Currently only "knows about" $[2]$ as it "forgets" the first occurrence of $1,2,$ and $3$ assert$($fv$.$size$()==1)$; $//$ Should be $1$ fv$.$push$\_$back$(1)$; $//$ now holds $[2,1]$ as this is the second time seeing a $1$ assert$($fv$.$at$(0)==2)$; $//$ returns a $2$ as that is the value in index $0$ in this forgetful vector assert$($fv$.$at$(1)==1)$;