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.

解题思路：

    类似快排的写法，两头逼近。去掉不满足条件的字符，然后做个比較就可以.

解题代码:

class Solution {
public:
    bool isPalindrome(string s)
    {
        bool res = true;
        int i = 0, j = s.size() - 1;    
        while ( res && i < j)
        {
            while (i < j && !(isdigit(s[i]) || isalpha(s[i])))
                ++i;
            while (i < j && !(isdigit(s[j]) || isalpha(s[j])))
                --j;
            if (i < j)
            {
                res = s[i] == s[j] || abs(s[i] - s[j]) == 32;
                ++i, --j;
            }
        }
        return res;
    }
};