Take 2 strings s1 and s2 including only letters from ato z. Return a new sorted string, the longest possible, containing distinct letters, each taken only once - coming from s1 or s2.

Two to One

Example: 
// a = "xyaabbbccccdefww"
// b = "xxxxyyyyabklmopq"
// longest(a, b) -> "abcdefklmopqwxy"

// a = "abcdefghijklmnopqrstuvwxyz"
// longest(a, a) -> "abcdefghijklmnopqrstuvwxyz"


function longest(s1, s2) {
  return (s1 + s2).split('').filter(onlyUnique).sort().join('')
}

function onlyUnique(value, index, self) { 
    return self.indexOf(value) === index;
}