LeetCode – Refresh – Valid Palindrome

class Solution {
public:
    string preProcess(string s) {
        string result;
        for (int i = 0; i < s.size(); i++) {
            if ((s[i] >= 'a' && s[i] <= 'z') ||
                (s[i] >= '0' && s[i] <= '9')) {
                result += s[i];    
            } else if (s[i] >= 'A' && s[i] <= 'Z') {
                result += s[i] - 'A' + 'a';
            }
        }
        return result;
    }
    bool isPalindrome(string s) {
        s = preProcess(s);
        int start = 0, end = s.size()-1;
        while(start < end) {
            if (s[start++] != s[end--]) return false;
        }
        return true;
    }
};