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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.\

题意：给定一个字符串，确定它是否是回文，只考虑字母数字字符；

思路：从头遍历和从尾部遍历同时进行；

跳过不是字符的；

注意题意，忽略大小写，即a==A

1. class Solution  

2. {  

3. public:  

4.     bool isPalindrome(string s)  

5.     {  

6.         if (s.empty())  

7.         {  

8.             return true;  

9.         }  

10.         int start = 0;  

11.         int end = s.size() - 1;  

12.         while (start<end)  

13.         {  

14.             //忽略非字符和非数字的  

15.             while (!((s[start] >= 'a' && s[start] <= 'z')  

16.                 || (s[start] >= 'A' && s[start] <= 'Z')  

17.                 || (s[start] >= '0' && s[start] <= '9'))  

18.                 && start<s.size())  

19.             {  

20.                 start++;  

21.             }  

22.   

23.             while (!((s[end] >= 'a' && s[end] <= 'z')  

24.                 || (s[end] >= 'A' && s[end] <= 'Z')  

25.                 || (s[end] >= '0' && s[end] <= '9'))  

26.                 && end >= 0)  

27.             {  

28.                 end--;  

29.             }  

30.   

31.             if (s[start] == s[end] || s[start] + 32 == s[end] || s[start] - 32 == s[end])  

32.             {  

33.                 start++;  

34.                 end--;  

35.             }  

36.             else  

37.             {  

38.                 return false;  

39.             }  

40.         }  

41.         return true;  

42.     }  

43. };