You need to write a class, named "Penultimate". This class has just one method $($member function$)$ named push$\_$back, that takes a string and returns a string. The string returned by push$\_$back is the string that was provided to push$\_$back in the previous call of the method $($or the string provided to the constructor if this is the first call to push$\_$back.

Example:

Penultimate p$("$first$")$;

string s $=$ p$.$push$\_$back$("$second$")$; $//$ s is "first"

p$.$push$\_$back$("$Third$")$; $//$ returns "second"

p$.$push$\_$back$("$last$")$; $//$ returns "Third"