bool isDigit(char c)

1. Design a function
   bool isDigit(char c)
   which returns true when c is a decimal digit character '0', '1', '2', ..., '9'.
2. You may test your function with the following main program:

   
   int main()
   {
       char str[] = "1A2B.g0v";
       for (int i=0; i< strlen(str); i++)
           if ( isDigit(str[i]) )
               cout << "True" << endl;
           else
               cout << "False" << endl;
       return 0;
   }