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.

先预处理一下，转化成所需要的类型，只要用简单的回文判断方法就完了。

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.size()==0) return true;
        char *str=new char [s.size()];
        int k=0;
        for(int i=0;i<s.size();i++){
        	if(s[i]>='a'&&s[i]<='z') str[k++]=s[i];
        }
        str[k]='\0';
        int st=0,ed=k;
        while(str[st++]==str[ed--]&&str<ed);
        delete []str;
        if(st>=ed) return true;
        else return false;
    }
};