125. 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.

去掉 non-alphanumeric 的字符，然后忽略大小写判断是不是回文串。

two pointers

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