Valid Palindrome

class Solution {public:    bool isPalindrome(string s) {        if(s.empty())   return true;         int lhs=0,rhs=s.size()-1;        while(lhs<rhs)        {            while((lhs<=s.size()-1)&&(!isalnum(s[lhs])))            {                lhs++;            }            while((rhs>=0 && (!isalnum(s[rhs]))))            {                rhs--;            }            if((lhs<rhs) && (s[lhs]!=s[rhs]) && (abs(s[lhs]-s[rhs])!='a'-'A'))            {                return false;            }            lhs++;            rhs--;        }        return true;    }};