NSString-----------------------------------------------------------------------------------------------------------------------
基本操作：
>> -(int) length
>> -(const char*) cString
>> -(const char*) UTF8String 以UTF-8形式返回字符串，Mac OS X所有的底层系统都能处理以UTF-8编码的char *
>> -(NSString *) stringByAppendingString:(NSString *) aString
构建并调试（Build and debug） #-Y
处理字符串的字串
-(NSRange) rangeOfString:(NSString *) aString
-(NSString *) substringFromIndex: (unsigned) index
-(NSString *) substringToIndex: (unsigned) index
-(NSString *) substringWithRange:(NSRange) range
操作文件
-(NSString *) lastPathComponent
-(NSString *) pathExtension
-(NSString *) stringByStandardizingPath
-(NSString *) stringWithContentsOfFile:(NSString *) path
-(BOO) writeToFile:(NSString *) path atomically:(BOOL) flag
NSMutableString--------------------------------------------------------------------------------------------------------------
-(void) appendString:(NSString *) aString
-(void) deleteCharactersInRange:(NSRange) range
-(void) insertString:(NSString *) aString atIndex:(unsigned) index
