Valid Palindrome [leetcode]

class Solution {

public:

    bool isPalindrome(string s) {

        if(s=="") return true;

        int i=0;

        string s1;

        while(i<s.length()){

            if(s[i]>='A' && s[i]<='Z') s1+=s[i]+32;

            else if(s[i]>='a' && s[i]<='z') s1+=s[i];

            else if(s[i]>='0' && s[i]<='9') s1+=s[i];

            i++;

        }

        string s2=s1;

        reverse(s1.begin(),s1.end());

        return s2==s1;        

    }

};