The suggested solution to this problem has given a clear idea. The tricky part of this problem is to handle all the edge cases carefully and write a clean code.

The following code should be self-explanatory. Note that the use of toupper avoid some messy if-else statements.

1 class Solution { 2 public: 3     bool isPalindrome(string s) { 4         int l = 0, r = s.length() - 1; 5         while (l < r) { 6             while (l < r && !isalnum(s[l])) l++; 7             if (l >= r) break; 8             while (r > l && !isalnum(s[r])) r--; 9             if (toupper(s[l++]) != toupper(s[r--]))10                 return false;11         }12         return true;13     }14 };