给定一个字符串，验证它是否是回文串，只考虑字母和数字字符，可以忽略字母的大小写。输入: "A man, a plan, a canal: Panama" 输出: true

def isPalindrome(s):
    s1=''
    for i in s:
        if (ord(i)>=48 and ord(i)<=57) or (ord(i)>=65 and ord(i)<=90) or (ord(i)>=97 and ord(i)<=122):
            s1+=i.lower()
    s2=''.join(list(reversed(s1)))
    if s1==s2:
        return True
    else:
        return False