Valid Palindrome

class Solution {
public:
    bool isPalindrome(string s) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        int i=0, j=s.size()-1;
        while (i<j) {
            while (i<j && (!((s[i]>='a' && s[i]<='z') || (s[i]>='A' && s[i]<='Z') || (s[i]>='0' && s[i]<='9')))) {
                i++;
            }
            if (i>=j) return true;
            while (!((s[j]>='a' && s[j]<='z') || (s[j]>='A' && s[j]<='Z') || (s[j]>='0' && s[j]<='9'))) {
                j--;
            }
            if (!(s[i]==s[j] || s[i]==s[j]-'A'+'a' || s[i]-'A'+'a'==s[j])) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
};