[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.

思路：最一般的遍历+比较。注意题目要求和字母大小写问题。

　　　时间复杂度O(n),空间复杂度O(1)

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty()) return true;
        
        vector<char> cvec;
        for (string::size_type i = 0, j = s.size() - 1; i < j;) {
            if (!isalnum(s[i])) {
                ++i;
                continue;
            }
            if (!isalnum(s[j])) {
                --j;
                continue;
            }
            if (isdigit(s[i]) && s[i] != s[j]) {
                return false;
            }
            if (isalpha(s[i]) && (toupper(s[i]) != toupper(s[j]))) {
                return false;
            }
            ++i;
            --j;
        }
    
        
        return true;
    }
};