find the next two chars need to be compared.

compare these two ignoring cases.

```
bool isValid(char c) {
return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9');
}
bool isPalindrome(string s) {
int start = 0, end = s.size() - 1;
while (start < end) {
if (!isValid(s[start])) { start ++; continue; }
if (!isValid(s[end])) { end --; continue; }
if (s[start] != s[end] && abs(s[start] - s[end]) != 32) return false;
start ++;
end --;
}
return true;
}
```