leetcode: 125. valid palindrome

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if s == '': return True

        cs = 'abcdefghijklmnopqrstuvwxyz0123456789'
        is_char = lambda c: c in cs
        to_sstr = lambda s: ''.join([ c if is_char(c) else '' for c in s])

        sl = s.lower()
        ss = to_sstr(sl)

        for i in range(len(ss)/2):
            if ss[i] != ss[len(ss)-1-i]:
                return False
        return True

class Solution(object):
    cs = 'abcdefghijklmnopqrstuvwxyz0123456789'

    def is_char(self, c):
        return c in self.cs

    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if s == '' or len(s) == 1: return True
        s = s.lower()

        i = 0
        j = len(s) - 1
        flag = True

        while i < j:
            ci = s[i]
            if not self.is_char(ci):
                i += 1
                continue

            cj = s[j]
            if not self.is_char(cj):
                j -= 1
                continue

            if ci == cj:
                i += 1
                j -= 1
                continue
            else:
                flag = False
                break

        return flag