leetcode---Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,

“A man, a plan, a canal: Panama” is a palindrome.

“race a car” is not a palindrome.

class Solution {
public:
bool isPalindrome(string s)
{
char ss[100000];
int len = s.length();
int index = 0;
for(int i=0; i<len; i++)
{
if(isalpha(s[i]))
ss[index++] = toupper(s[i]);
if(isdigit(s[i]))
ss[index++] = s[i];
}
int l = 0;
int r = index - 1;
while(l < r)
{
if(ss[l] != ss[r])
return false;
l++;
r--;
}
return true;
}
};