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 };