[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.

 1 class Solution {

 2 public:

 3     bool isPalindrome(string s) {

 4         int count = 0;

 5         for (int i = 0; i < s.length(); ++i) {

 6             if (s[i] >= 'A' && s[i] <= 'Z') {

 7                 s[i-count] = s[i] - 'A' + 'a';

 8             } else if (s[i] >= 'a' && s[i] <= 'z' || s[i] >= '0' && s[i] <= '9') {

 9                 s[i-count] = s[i];

10             } else {

11                 count++;

12             }

13         }

14         s.resize(s.length() - count);

15         int a = 0, b = s.length() - 1;

16         while (a < b) {

17             if (s[a] != s[b]) {

18                 return false;

19             }

20             a++; b--;

21         }

22         return true;

23     }

24 };