Scala, list append don't work inside recursive function - scala

Question

So I was working on algorithm that would take a binary tree:
sealed trait BT
case object Empty extends BT
case class Node(val x: Int, val left: BT, val right: BT) extends BT
and return binary tree but without duplicates with help of recursive dfs to search the tree.
def removeRecurring_dfs(bt: BT, found: List[Int]): BT = {
def dfs_helper(tree: BT): BT = {
tree match
case Empty => Empty
case Node(x, _, _) if found.contains(x) => dfs_helper(rR(tree))
case Node(x, Empty, Empty) =>
x::found
tree
case Node(x, left, Empty) =>
x::found
Node(x, dfs_helper(left), Empty)
case Node(x, Empty, right) =>
x::found
Node(x, Empty, dfs_helper(right))
case Node(x, left, right) =>
x::found
Node(x, dfs_helper(left), dfs_helper(right))
}
def rR(tree: BT): BT = removeNode(tree, findPath(tree))
dfs_helper(bt)
}
List 'found' is initially empty. 'removeNode' and 'findPath' are functions that help me achieve my goal. After some testing I found out that
x::found
line never works, even when case with that line is triggered. With some println action it turned out that pattern matching works as I intented.
I have started playing with scala recently and i don't understand what could cause such behavior

Answer 1

As has been pointed out, List is immutable so x :: found creates a new List with the element x prepended to the found list.
The new List has to be saved/sent somewhere, otherwise it is lost and garbage collected.
I don't know what your findPath() and removeNode() methods are supposed to do, so it's a bit hard to say, but it looks like this is what you're trying to accomplish. (Scala-3 syntax)
sealed trait BT
case object Empty extends BT
case class Node(x: Int, left: BT, right: BT) extends BT
def removeRecurring_dfs(bt: BT): BT =
def dfs_helper(tree:BT, found:Set[Int]): (BT,Set[Int]) = tree match
case Empty => (Empty, found)
case Node(x, left, right) =>
if found(x) then
dfs_helper(rR(tree), found)
else
val (lftNode, moreFound) = dfs_helper(left, found + x)
val (rgtNode, totalFound) = dfs_helper(right, moreFound)
(Node(x, lftNode, rgtNode), totalFound)
def rR(tree: BT): BT = removeNode(tree, findPath(tree))
dfs_helper(bt, Set())._1