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;        
    }
};

思想：利用string反转，如果跟反转之前一样，说明是回文数字