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.

分析：判断一个字符串是否为回文串，只要判断前后对应位置的字符是否相等即可，在此之前，此题需要做一些处理比如去除非字符数字字符和将字符变为小写等。

class Solution {
public:
    bool isPalindrome(string s) {
        int n = s.length();
        if(n == 0) return true;
        
        int left = 0, right = n-1;
        while(left < right){
            if(!isalnum(s[left]))left++;
            else if(!isalnum(s[right]))right--;
            else if(tolower(s[left]) != tolower(s[right])) return false;
            else{
                left++;
                right--;
            }
        }
        return true;
    }
};