[LeetCode]Valid Palindrome

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.

这道题是判断一个字符串是否是回文字符串，有几个注意点：
1. 不区分大小写
2. 非数字和字母的字符忽略
3. 空字符串也认为是回文字符串

所以最一般的例子是形如，；‘’\n \t的字符串，被视为空字符串，也就是回文的。

本来准备使用额外的空间来对字符串做个预处理，后来想想，还是不必了，头尾同时遍历的时候做个判断就可以了。遇到非数字字母的字符跳过，否则判断两字符是否“相等”：互为大小写或者完全相同。我处理的方式是两者相减，判断绝对值是否是0或者32.
下面贴上代码：

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty())
            return true;
        int len = s.length();
        int i = 0, j = len - 1;
        while (i <= j){
            while (i<len&&!isalnum(s[i]))
                i++;
            while (j>=0&&!isalnum(s[j]))
                j--;
            if (i > j)
                return true;
            int num = abs(s[i] - s[j]);
            if (num != 0 && num != 32)
                return false;
            i++;
            j--;
        }
        return true;
    }
};