125. Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: “A man, a plan, a canal: Panama”
Output: true

Example 2:

Input: “race a car”
Output: false

思路：遍历字符串，去掉非字母数字的，倒序检查是否相等

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        plaindrome = list(s)
        newstr = ""
        for i in plaindrome:
            if i.isalnum():
                i = i.lower()
                newstr+=i
        return newstr==newstr[::-1]

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

        i = -1
        j = len(s)
        while True:
            i += 1
            j -= 1
            if i > j:
                return True
            while i < j:
                if not s[i].isalnum():
                    i += 1
                else:
                    break
            while i < j:
                if not s[j].isalnum():
                    j -= 1
                else:
                    break
            if s[i].lower() != s[j].lower():
                return False

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        cleanlist = [c for c in s.lower() if c.isalnum()]
        return cleanlist == cleanlist[::-1]