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:

这道题是验证是否是会问字符串，但是字符串中加入了空格和非字母数字的字符，增加了难度。只需要建立两个指针，l和r分别从头和尾开始遍历整个字符串，遇到非数字字母的字符就跳过，继续往下找。等左右指针都找到字符时就比较这两个字符是否相等，不想等返回false，若相等接着比较下面两个找到的字符。在最开始将字母全部转换成小写的。

var isPalindrome = function(s) {
    if(! s.length) return true;
    s=s.toLowerCase()
    var l=0;
    var r=s.length-1;
    while(l<r){
        if(!isAlphaNum(s[l]))l++;
        else if(!isAlphaNum(s[r]))r--;
        else if(s[l]!=s[r]) return false;
        else{
            l++;
            r--;
        }     
    }
    return true;
    
    function isAlphaNum(ch){
        if(ch>='a' && ch<='z') return true;
        if(ch>='A' && ch<='Z') return true;
        if(ch>='0' && ch<='9') return true;
        return false;
    }
};