Read a $3-$character string from input into variable userString. Declare a Boolean variable allDigits and assign allDigits with true if userString only contains digits. Otherwise, assign allDigits with false

Ex: If the input is $433,$ then allDigits is assigned with true, so the output is:

String is valid

Ex: If the input is $905,$ then allDigits is assigned with false, so the output is:

String is not valid

Note. Use getline $($cin$,$ userString$)$ to read the entire line from input into userstring.