leetcode #125 in cpp

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.

Code:

class Solution {
public:
bool isPalindrome(string s) {
int i = 0;
int j = s.length()-1;
while(i<=j){
while( i<s.length() && !isalnum(s[i])){
i++;
}
while(j>=0 && !isalnum(s[j])){
j--;
}
if(i <= j){
if(tolower(s[i]) == tolower(s[j])){
i++;
j--;
}else{
return false;
}
}
}
return true;
}
};