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;    }};