[LeetCode 题解]: Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

主要考察， cctype.h 的使用：

可能用到的函数： tolower, toupper, isalpha, isdigit, isalnum。

class Solution {
public:
    bool isPalindrome(string s) {
        int left=0,right =s.size()-1;
        while(left<right)
        {
            if(!isalnum(s[left]))    
                left++;
            else if(!isalnum(s[right]))
                right--;
            else
            {
                if(tolower(s[left])==tolower(s[right]))
                {
                    left++;
                    right--;
                }
                else
                    return false;
            }
        }
        return true;        
    }
};

可能用到的函数： tolower, toupper, isalpha, isdigit, isalnum。

补充知识:  cctype 库文件(参考cplusplus: http://www.cplusplus.com/reference/cctype/ )

函数名	function description	函数说明
isalnum	Check if character is alphanumeric (function )	查看参数是否为字符或数字
isalpha	Check if character is alphabetic (function )	查看参数是否为字符
isblank	Check if character is blank (function )	查看参数是否为空格
iscntrl	Check if character is a control character (function )	查看参数是否为控制字符
isdigit	Check if character is decimal digit (function )	查看参数是否为十进制数字
isgraph	Check if character has graphical representation (function )	查看参数是否为可显示字符
islower	Check if character is lowercase letter (function )	查看参数是否为可打印字符
isprint	Check if character is printable (function )	查看参数是否为小写字符
ispunct	Check if character is a punctuation character (function )	查看参数是否为标点符号
isspace	Check if character is a white-space (function )	查看参数是否为空字符
isupper	Check if character is uppercase letter (function )	查看参数是否为大写字符
isxdigit	Check if character is hexadecimal digit (function )	查看参数是否为十六进制数字

具体的ASCII 与 cctype各函数的对应关系如下表： 

ASCII values

characters

iscntrl

isblank

isspace

isupper

islower

isalpha

isdigit

isxdigit

isalnum

ispunct

isgraph

isprint

x00 .. 0x08

NUL, (other control codes)

x

0x09

tab (' ')

x

x

x

0x0A .. 0x0D

(white-space control codes:'f','v',' ',' ')

x

x

0x0E .. 0x1F

(other control codes)

x

0x20

space (' ')

x

x

x

0x21 .. 0x2F

!"#$%&'()*+,-./

x

x

x

0x30 .. 0x39

123456789

x

x

x

x

x

0x3a .. 0x40

:;<=>?@

x

x

x

0x41 .. 0x46

ABCDEF

x

x

x

x

x

x

0x47 .. 0x5A

GHIJKLMNOPQRSTUVWXYZ

x

x

x

x

x

0x5B .. 0x60

[]^_`

x

x

x

0x61 .. 0x66

abcdef

x

x

x

x

x

x

0x67 .. 0x7A

ghijklmnopqrstuvwxyz

x

x

x

x

x

0x7B .. 0x7E

{|}~

x

x

x

0x7F

(DEL)

x

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