Valid Palindrome

class Solution {

public:

bool isPalindrome(string s) {

transform(s.begin(),s.end(),s.begin(),::tolower);

auto left=s.begin();

auto right=prev(s.end());

while(left<right){

if(!::isalnum(*left)) ++left;

else if( !::isalnum(*right)) --right;

else if(left!=right)

return false;

else{

++left;

--right;

}

}

return true;

}

};