Leetcode Problem.125—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.

My C++ solution！

string characterOnly(string str)
{
	int len=str.length();
	string r;
	for(int i=0;i<len;i++)
	{
		if(('A'<=str[i]&&str[i]<='Z')||('a'<=str[i]&&str[i]<='z')||('0'<=str[i]&&str[i]<='9'))
			   r.append(1,toupper(str[i]));
		else 
			continue;
	}
	return r;
}

bool isPalindrome(string s)
{
	s=characterOnly(s);
	int len=s.length();
	for(int i=0;i<(len+1)/2;i++)
	{
		if(s[i]!=s[len-i-1])
			return false;
	}
	return true;
}