valid-palindrome——判断带符号数字字母的字符串是否为回文

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.

字符串为空时判断为回文，大小写不区分可确定相等，数字与字母不同。

 class Solution {
 public:
     bool isPalindrome(string s) {
         int n=s.length();
         if(n==) return true;
         int i=,j=n-;
         while(i<j){
             if(!isCharacters(s[i])){
                 i++;
                 continue;
             }
             if(!isCharacters(s[j])){
                 j--;
                 continue;
             }

             int left=,right=,leftsig=,rightsig=;
             left=(s[i]>=''&&s[i]<='')?s[i]-'':((s[i]>='a'&&s[i]<='z')?s[i]-'a':s[i]-'A');
             right=(s[j]>=''&&s[j]<='')?s[j]-'':((s[j]>='a'&&s[j]<='z')?s[j]-'a':s[j]-'A');
             leftsig=(s[i]>=''&&s[i]<='')?:;
             rightsig=(s[j]>=''&&s[j]<='')?:;
             if(left!=right||leftsig!=rightsig)
                 return false;
             i++;
             j--;
         }
         return true;
     }
     bool isCharacters(char c){
         if((c>='a'&&c<='z')||(c>='A'&&c<='Z')||(c>=''&&c<=''))
             return true;
         else
             return false;
     }
 };