could improve 125. Valid Palindrome

class Solution {
public:
bool isPalindrome(string s) {
if(s.size()==0||s.size()==1) return true;
string s2,s1;
for(int i=0,j=s.size()-1;i<s.size()&&j>=0;i++,j--)
{
if((s[i]>='0'&&s[i]<='9')||(s[i]>='a'&&s[i]<='z'))
s1.push_back(s[i]);
if(s[i]>='A'&&s[i]<='Z') s1.push_back(s[i]-'A'+'a');

if((s[j]>='0'&&s[j]<='9')||(s[j]>='a'&&s[j]<='z'))
s2.push_back(s[j]);
if(s[j]>='A'&&s[j]<='Z') s2.push_back(s[j]-'A'+'a');
}
if(s2==s1) return true;
else return false;
}
};