/*
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.

给出一字符串，判断它是否回文。只认字母，以及忽略大小写。
*/
var isAlphaNum = function(s){
  return /[a-z0-9]/i.test(s);
};

var isPalindrome = function(s){
  var trimS = s.trim();
  if(trimS === ''){
    return true;
  }
 
  var left = 0;
  var right = trimS.length - 1;
  while(left < right){
    if(!isAlphaNum(trimS[left])){
      left++;
    }
    else if(!isAlphaNum(trimS[right])){
      right--;
    }
    else if(trimS[left].toUpperCase() !== trimS[right].toUpperCase()){
      return false;
    }
    else{
      left++;
      right--;
    }
  }
  return true;
};


console.log(
  isPalindrome('A man, a plan, a canal: Panama')
);