leetcode125. 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.

验证回文字符串是比较常见的问题，所谓回文，就是一个正读和反读都同样的字符串，好比“level”或者“noon”等等就是回文串。可是这里，加入了空格和非字母数字的字符，增长了些难度，但其实原理仍是很简单：只须要创建两个指针，left和right, 分别从字符的开头和结尾处开始遍历整个字符串，若是遇到非字母数字的字符就跳过，继续往下找，直到找到下一个字母数字或者结束遍历，若是遇到大写字母，就将其转为小写。等左右指针都找到字母数字时，比较这两个字符，若相等，则继续比较下面两个分别找到的字母数字，若不相等，直接返回false.

时间复杂度为O(n), 代码以下：

var isPalindrome = function(s) {
       var n = s.length;
       if( n <= 1){
           return true
       }
       var left = 0;
       var right = n -1;
       while(left < right){
           if(!isAlphanumeric(s[left])){
               left++
           }else if(!isAlphanumeric(s[right])){
               right--
           }else if(s[left].toLowerCase() !== s[right].toLowerCase()){
               return false
           }else{
               left++
               right--
           }               
       }
       return true    
};
function isAlphanumeric(a){
   var c = a.charCodeAt(0)
   if( c >= 48 &&  c<=57){//0-9
       return true
   }
   if( c >= 65 &&  c<= 90){//A-Z
       return true
   }
   if( c >= 97 &&  c<= 122){//a-z
       return true
   }
   return false
}