[Aetae]

function fn(arr){
  var obj = Object.create(null), i = arr.length, max = 0;
  while(i--){
    if(arr[i] > max){
      while(++max < arr[i])
        obj[max] = true;
    }else{
      delete obj[arr[i]];
    }
  }
  return Object.keys(obj);
}