Valid Palindrome

要注意两点：
1.判断是否是大小写字母或数字，统一将大小写字母变成小写字母
2.isalnum()函数判断是大小写字母或者数字，lower()函数将大写字母变成小写字

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """

        new_s = [] #这里如果以字符串保存会超时，只好使用数组
        for c in s:
            if c >= 'a' and c <= 'z' or c >= 'A' and c <= 'Z' or c >= '0' and c <= '9':
            #if c.isalnum():
                new_s.append(c.lower())

        length = len(new_s)
        for i in range(0, length/2):
            if new_s[i] != new_s[length-i-1]:
                return False
        return True