【leetcode】Valid Palindrome

题目简述：

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.

解题思路：

class Solution:
    # @param s, a string
    # @return a boolean
    def isPalindrome(self, s):
        if s == '':
            return True

        s = s.strip().lower()
        t = ''
        for i in s:
            if (i <= '9' and i >= '0') or (i <= 'z' and i >= 'a'):
                t += i

        l = len(t)
        for i in range(l/2):

            if t[i] != t[l-i-1]:
                return False
        return True