DFS in Swift not terminating - swift

Question

My implementation of a BFS works fine:
func bfsReturnVals() -> [Int] {
// bfs uses a queue.
var queue = [Node]()
var queueVals = [Int]()
queue.append(self)
while let head = queue.first {
if let lft = head.left {
queue.append(lft)
}
if let rgt = head.right {
queue.append(rgt)
}
queueVals.append(queue[0].data)
queue.removeFirst()
}
return queueVals
}
But I usually code a DFS recursively. Now my similar implementation of DFS does not terminate
func dfsReturnVals() -> [Int] {
var stack = [Node]()
var queueVals = [Int]()
stack.append(self)
while let tail = stack.last {
if let lft = tail.left {
stack.append(lft)
}
if let rgt = tail.right {
stack.append(rgt)
}
queueVals.append(stack[stack.count - 1].data)
stack.removeLast()
}
return queueVals
}
I can't work out why. Shouldn't removeLast() work just like removeFirst()?
My node class is as follows:
class Node : CustomStringConvertible {
var description : String {return "\(self.data)" }
var data : Int
var left: Node?
var right: Node?
init(_ data: Int) {
self.data = data
}
func insert(_ data: Int) {
if data < self.data {
if let lft = self.left {
lft.insert(data)
} else {
let newNode = Node(data)
left = newNode
}
} else {
if let rgt = self.right {
rgt.insert(data)
} else {
let newNode = Node(data)
right = newNode
}
}
}
}

Answer 1

When you remove the last item from the stack, you're removing the items you just pushed on. In your bfs implementation, remove first will grab the intended parent node regardless of the fact that you do it at the end of the loop, because you add the children to the end of the queue. You should move the operation to remove the expanded node from your stack to before you push new nodes on.