用了几个Character handling functions，简化了代码。挺实用的。
<cctype> (ctype.h)
Character handling functions
This header declares a set of functions to classify and transform individual characters.
Functions
These functions take the int equivalent of one character as parameter and return an int that can either be another character or a value representing a boolean value: an int value of 0 means false, and an int value different from 0 represents true.
There are two sets of functions:
Character classification functions
They check whether the character passed as parameter belongs to a certain category:
isalnum
Check if character is alphanumeric (function )
isalpha
Check if character is alphabetic (function )
isblank
Check if character is blank (function )
iscntrl
Check if character is a control character (function )
isdigit
Check if character is decimal digit (function )
isgraph
Check if character has graphical representation (function )
islower
Check if character is lowercase letter (function )
isprint
Check if character is printable (function )
ispunct
Check if character is a punctuation character (function )
isspace
Check if character is a white-space (function )
isupper
Check if character is uppercase letter (function )
isxdigit
Check if character is hexadecimal digit (function )
Character conversion functions
Two functions that convert between letter cases:
tolower
Convert uppercase letter to lowercase (function )
toupper
Convert lowercase letter to uppercase (function )