[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.

class Solution {
public:
    bool isPalindrome(string s) {
        int count = 0;
        for (int i = 0; i < s.length(); ++i) {
            if (s[i] >= 'A' && s[i] <= 'Z') {
                s[i-count] = s[i] - 'A' + 'a';
            } else if (s[i] >= 'a' && s[i] <= 'z' || s[i] >= '0' && s[i] <= '9') {
                s[i-count] = s[i];
            } else {
                count++;
            }
        }
        s.resize(s.length() - count);
        int a = 0, b = s.length() - 1;
        while (a < b) {
            if (s[a] != s[b]) {
                return false;
            }
            a++; b--;
        }
        return true;
    }
};