LeetCode 125. Valid Palindrome

这个题目只要注意大小写问题即可解决，而且我们只关注的是字母和数值

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.

class Solution {
public:
    bool isPalindrome(string s) {
        int start = 0, end = s.size() - 1;
        while (start < end)
        {
            if ('a' <= s[start] && s[start] <= 'z' || 'A' <= s[start] && s[start] <= 'Z' ||
                '0' <= s[start] && s[start] <= '9')
            {
                bool tag = true;
                while (tag)
                {
                    if ('a' <= s[end] && s[end] <= 'z' || 'A' <= s[end] && s[end] <= 'Z' ||
                        '0' <= s[end] && s[end] <= '9')
                    {
                        if (s[start] == s[end]||s[start]==toupper(s[end])||s[start]==tolower(s[end]))
                        {
                            ++start;
                            --end;
                            tag = false;
                            
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        --end;
                        tag = true;
                    }
                }
            }
            else
                ++start;
        }
        return true;
    }
};