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.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

   bool alphanum(char s)  
   {  
     if (s>='0' && s<='9')return true;  
     if (s>='a' && s<='z')return true;  
     if (s>='A' && s<='Z')return true;  
     return false;  
   }  



   bool isPalindrome(string s) {  
     int i = 0, j = s.length()-1;  
     if(j<1)return true;  
     while(i<j)  
     {  
       if(s[i]>='A' && s[i]<='Z')s[i] += 32;  
       if(s[j]>='A' && s[j]<='Z')s[j] += 32;  
       if(!alphanum(s[i])){  
         i++;  
         continue;  
       }  
       else if(!alphanum(s[j]))  
       {  
         j--;  
         continue;  
       }  
       if(s[i] != s[j])return false;  
       i++;  
       j--;  
     }  
     return true;  
   }