LeetCode125：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

LeetCode：

分两步：第一步判断字符是否是数字或者字母，第二步判断是否回文。

判断字符是否是数字或者字母：如果字符在65-90（A-Z）或者97-122（a-z）或者48-57（0-9）之间，则返回true，否则false。判断是否回文：设置两个指针i=0和j=s.lenght()-1，各自从两头遍历，如果二者都是数字或字母，变小写比较大小；如果不是，就+1或-1跳过。

class Solution(object):
    def isPalindrome(self, s):
        """
        :type s: str
        :rtype: bool
        """
        if len(s) == 0:
            return True

        def isnumalpha(c):
            if (c>='a' and c<='z') or (c>='A' and c<='Z') or (c>='0' and c<='9'):
                return True
            else:
                return False

        start = 0
        end = len(s) - 1
        while start <= end:
            if isnumalpha(s[start]) and isnumalpha(s[end]):
                if s[start].lower() != s[end].lower():
                    return False
                start += 1
                end -= 1
            elif isnumalpha(s[start]) == False:
                start += 1
            elif isnumalpha(s[end]) == False:
                end -= 1 
        return True