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;    }};