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.

比较简单。

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.size();
        if (len == 0) {
            return true;
        }

        --len;
        for (int i = 0; i < len; ++i, --len) {
            while (!isValidChar(s[i]) && i < len) {
                ++i;
            }
            while (!isValidChar(s[len]) && i < len) {
                --len;
            }
            if (i >= len) {
                return true;
            }
            else if (tolower(s[i]) != tolower(s[len])) {
                return false;
            }
        }
        
        return true;
    }
    
    bool isValidChar(char ch) {
        return (ch >= 'a' && ch <= 'z' ||
                ch >= 'A' && ch <= 'Z' ||
                ch >= '0' && ch <= '9');
    }
};

==================第二次=======================

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty()) {
            return true;
        }
        
        int j = s.size() - 1;
        for (int i = 0; i < j; ++i, --j) {
            while (i < j && !isalnum(s[i])) {
                ++i;
            }
            while (i < j && !isalnum(s[j])) {
                --j;
            }
            
            if (tolower(s[i]) != tolower(s[j])) {
                return false;
            }
        }
        
        return true;
    }
};