Insert an element between each two adjacent elements of Seq

Try for comprehension:

for(i <- list; p <- List(0, i)) yield p

However you must somehow remove the first element (it yields: 0,1,0,2,0,3), either by:

(for(i <- list; p <- List(0, i)) yield p).tail

or:

list.head :: (for(i <- list.tail; p <- List(0, i)) yield p)

Here is another approach:

def intersperse[E](x: E, xs:Seq[E]): Seq[E] = (x, xs) match {
    case (_, Nil)     => Nil
    case (_, Seq(x))  => Seq(x) 
    case (sep, y::ys) => y+:sep+:intersperse(sep, ys)
}

which is safe over empty Seqs too.