3.1ValidPalindrome

Notes:
	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.
	Solution: traverse from both side.
	*/

思路：判断一个字符串是不是回文，看字符串是不是以中间位置对称，对称的话就是回文，否则不是。

#include <cctype> // needed for isalnum(), isupper() and tolower().
 /**** isalnum()用来判断是不是字符或数字，isupper() 判断是不是大写字母， tolower()将大写字母转换成小写字母*****/
class Solution {
public:
    bool isPalindrome(string s) {
        for (int i = 0, j = s.size() - 1; i < j; ++i, --j)
        {
            while (i < j && !isalnum(s[i])) i++;
            while (i < j && !isalnum(s[j])) j--;

            if (tolower(s[i]) != tolower(s[j]))
                return false;
        }
        return true;
    }
};