验证回文串

输入: "A man, a plan, a canal: Panama"
输出: true

输入: "race a car"
输出: false

/**
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function(s) {
    if (!s) return true
    let arr = [...s.toLowerCase()].filter(
        item => 'a' <= item && item <= 'z' 
        || '0' <= item && item <= '9')

    let left = 0, right = arr.length - 1
    while(left <= right) {
        if ( arr[left] !== arr[right]) {
            return false
        }
        left++
        right--
    }   
    return true 
};