Write a C++ program that converts letters of the alphabet into their corresponding digits on the telephone. The program should let the user enter letters repeatedly until a Q or a Z is entered. (Q and Z are the two letters that are not on the telephone.) An error message should be printed for any nonalphabetic character that is entered.

The letters and digits on the telephone have the following correspondence.

ABC = 2 DEF = 3 GHI = 4

JKL = 5 MNO = 6 PRS = 7

TUV = 8 WXY = 9

Here is an example:

Enter a letter : P

The letter P corresponds to 7 on the telephone.

Enter a letter : M

The letter M corresponds to 6 on the telephone.

Enter a letter : S

The letter S corresponds to 7 on the telephone.

Enter a letter : O

The letter O corresponds to 6 on the telephone.

Enter a letter : 2

Invalid letter. Enter Q or Z to quit.

Enter a letter : Q

Quit.