function isPalindrome(string) {
    return string.split('').reverse().join('') == string
}

function optimized(string) {

	var truthyString = '';

	for (var i=0; i<string.length;i++) {

		// we add some value to the truthy string. doesnt matter what value it is.
		// we break out of the iteration early if we do not find a match at the 
		// corresponding negative index in this optimized version

		if (string[string.length - i - 1] == string[i]) {
			truthyString += 'a'
		} else {
			break;
		}
	}

	// since we appended some dummy value we only care about length. If the lengths
	// are not the same they cannot possibly be equal. This version assumes case 
	// sensitive matching

	return truthyString.length == string.length;
}

function is_palindrome (str) {
    for (i = 0, i < str.length / 2; i++) {
       if (str[i - 1] != str[-i]) {
           return false
   }
   return true
}

function is_palindrome(str) {
    for (i = 1; i <= str.length / 2; i ++) {
         if (str[i - 1] != str.charAt(str.length - i)) {
               return false;
         }
     }
     return true 
}

// all evaluate to true:
is_palindrome("");
is_palindrome("a");
is_palindrome("abba");
is_palindrome("abcba");

// all evaluate to false:
is_palindrome("abca");
is_palindrome("abcab");

ispalindrome(s):
    l = 0
    r = len(s) - 1

    while l < r:
        if s[l] == s[r]:
            l = l + 1
            r = r - 1
        else:
            return False

    return True

def ispalindrome2(s):
    return reversed(s) == s

  function mostCommonString(input) {

    var unique = []

    var strings = input.split(/\s+/)

    strings.forEach(function(element) {
      unique.indexOf(element.toLowerCase()) == -1 ? unique.push(element.toLowerCase()) : void 0;
    })

    var duplicates = unique.filter(function(element) {

      var hasDuplicates = strings.filter(function(ele) {
        return ele == element.toLowerCase()
      })

      return hasDuplicates.length > 1
    })
    
    return duplicates.length > 1 ? duplicates : duplicates.join('')
  }

function(inputString) {
     if inputString is empty:
          return nil or null
     dictionary = {}
     current_word = "";
    
     for (char c in inputString)
         if (c is a letter):
               add c to current_word
         else if current_word not empty:
             if current_word not in dictionary:
                 dictionary[current_word] = 1
             else
                 dictionary[current_word] += 1
                 current_word = ""

     if current_word not empty:
             if current_word not in dictionary:
                 dictionary[current_word] = 1
             else
                 dictionary[current_word] += 1
                 current_word = ""
      
      loop through dictionary and return key with highest value
}