【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.

空串是回文串。

分析：首尾开始比较，不是字母的跳过，都是字母且相同继续比较，否则就是返回false。

 bool alphanumeric (char c)    {        return (c >= 'a' && c <= 'z' || c >= '0' && c <= '9');    }    void tolower(string &s)    {        for(int i = 0; i < s.size(); ++i)        if(s[i] <= 'Z' && s[i] >= 'A')        s[i] += 32;    }    bool isPalindrome(string s) {        if(s.size() <= 1) return true;        int i = 0;        int j = s.size() - 1;        tolower(s);        while(i <= j)        {            if(alphanumeric(s[i]) && alphanumeric(s[j]) && s[i] == s[j])            {++i; --j;}            else if(!alphanumeric(s[i])) ++i;            else if(!alphanumeric(s[j])) --j;            else  return false;        }        return true;    }