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.

代码：

class Solution {public:bool isPalindrome(string s){    int start=0,end=s.length()-1;while(start<end){while(start<end&&!isChar(s[start]))++start;while(start<end&&!isChar(s[end]))--end;if (start<end){if (!equal(s[start],s[end]))return false;++start;--end;}}return true;}bool isChar(char c){return (c>='0'&&c<='9')||(c>='a'&&c<='z')||(c>='A'&&c<='Z');}bool equal(char a,char b){if (a>='0'&&a<='9')return a==b;if (a>='A'&&a<='Z')a=a-'A'+'a';if (b>='A'&&b<='Z')b=b-'A'+'a';return a==b;}};