[LeetCode] 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.

解答：判断回文。忽略不是字母和数字的字符，同时忽略大小写　　

　　 时间复杂度O(n), 空间复杂度O(1)

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty()) return true;
        
        string::const_iterator left = s.begin();
        string::const_iterator right = s.end() - 1;
        
        while (left < right) {
            if (!isalnum(*left)) ++left;
            else if (!isalnum(*right)) --right;
            else if (tolower(*left) == tolower(*right)) {
                ++left;
                --right;
            } else {
                return false;
            }
        }
        return true;
    }
};