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==0) return true;
        int i=0,j=n-1;
        while(i<j){
            if(!isCharacters(s[i])){
                i++;
                continue;
            }
            if(!isCharacters(s[j])){
                j--;
                continue;
            }
                
            int left=0,right=0,leftsig=0,rightsig=0;
            left=(s[i]>='0'&&s[i]<='9')?s[i]-'0':((s[i]>='a'&&s[i]<='z')?s[i]-'a':s[i]-'A');
            right=(s[j]>='0'&&s[j]<='9')?s[j]-'0':((s[j]>='a'&&s[j]<='z')?s[j]-'a':s[j]-'A');
            leftsig=(s[i]>='0'&&s[i]<='9')?0:1;
            rightsig=(s[j]>='0'&&s[j]<='9')?0:1;
            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>='0'&&c<='9'))
            return true;
        else
            return false;
    }
};