LEET CODE PROBLEM :-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) {
        int l=s.length();
        char a1[l],c1=0;
        for(int i=0;i<l;i++)
    if((s.at(i)>=97&&s.at(i)<=122)||(s.at(i)>=65&&s.at(i)<=90)||(s.at(i)>=48&&s.at(i)<=57))
        {
                   a1[c1++]=s.at(i);
                   if(s.at(i)>=97)
                   a1[c1-1]-=32;
        }
                   a1[c1]='\0';
        int l1=0,h=c1-1;
        char a2[c1];
    for(int i=0,j=c1-1;i<c1;i++,j--)
         a2[i]=a1[j];
         a2[c1]=0;
    if(!strcmp(a1,a2))
    return true;
    return false;
}
};