leetcode_question_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.

bool ischar(char & ch)
    {
        if('a' <= ch && ch <= 'z') return true;
        else if('0' <= ch && ch <= '9') return true;
        else if('A' <= ch && ch <= 'Z'){ ch += 32; return true; }
        else return false;
    }

    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
    	int length = s.length();
        if(length <= 1)return true;

		int begin = 0;
		int end = length-1;
		while(begin < end)
		{
			while(begin < end && !ischar(s[begin]))
				++begin;
			while(begin < end && !ischar(s[end]))
				--end;

			if(s[begin++] != s[end--]) return false;
		}
		return true;
    }