Depth First Search Implementation - understanding swift code - swift

Question

I was going thru few tutorials for Tree DS and I found this code which is really confusing to understand. Please explain
public func forEachDepthFirst(visit: (TreeNode) -> Void) {
visit(self) // 1
children.forEach { // 2
$0.forEachDepthFirst(visit: visit)
}
}
}
Why do we have visit(self) here?
I see explanation here https://forums.raywenderlich.com/t/help-understanding-the-recursion-for-depth-first-traversal/56552/2 but its still not clear

Answer 1

Any recusive method has
1- base case : which ends the run and here it's
children.forEach // when children property is empty meaning a leaf node
2- recusive case
$0.forEachDepthFirst(visit: visit) // call the same method with it's children
Your method takes a closure / completion that's be called for every node inside the main root node
So suppose You have root
0
- 1
- 1.1 , 1.2 , 1.3
- 2
- 2.1 , 2.2 , 2.3
Here 0 node is called then when runnign your function
visit(0)
children.forEach { // = 1,2
for 0 > 1
visit(1)
children.forEach { // = 1.1,1.2,1.3
for 0 > 2
visit(2)
children.forEach { // = 2.1,2.2,2.3
Inner case
for 0 > 1 > 1.1
visit(1.1)
children.forEach { // end here as there is no childrens ( leaf node)
so on for 1.2,1,3
for 0 > 2 > 2.1 / 2.2 / 2.3 same as above case
How to call
your method is an instance method inside the tree so every node can call it , if you want to traverse nodes of 0 then do this
zeroNode.forEachDepthFirst { (item) in
print(item.name) // suppose node object has a name
}
Then you will get
0 , 1 , 1.1 , 1.2 , 1.3 , 2.1 , 2.2 , 2.3
And that's as you called visit(NodeObject) for the main node and recursively all it's childrens

Answer 2

Why do we have visit(self) here?
Because if we didn't, we would never actually do anything to any of the nodes on the tree!
Consider this tree:
n1 -> n2 -> n3 -> n4
We now call our method forEachDepthFirst on n1. If we didn't have visit(self), we would immediately call forEachDepthFirst on n2, which would call it on n3, which would call it on n4. And then we'd stop. But at no time would we have called visit, so we would have looped through every node in the tree without doing anything to those nodes.