Swift: Determine nodes in tree

Question

I abstracted my problem in a playground file to make discussion easier. My problem is that I need to figure out all values of nodes in a tree that has a minimum number of children. I do not get the recursion right. The result of my algorithm is [1,9], but should be: [1,3,9]. Any thoughts?

import Foundation

class TreeNode{

    var children: [TreeNode]?
    var value: Int

    init( value: Int, children: [TreeNode]?){
        self.value = value
        self.children = children
    }
}

let node11 = TreeNode(value: 11, children: nil)
let node10 = TreeNode(value: 10, children: nil)
let node9 = TreeNode(value: 9, children: [node10, node11])
let node8 = TreeNode(value: 8, children: nil)
let node7 = TreeNode(value: 7, children: [node9])
let node6 = TreeNode(value: 6, children: [node8])
let node5 = TreeNode(value: 5, children:nil)
let node4 = TreeNode(value: 4, children: [node7])
let node3 = TreeNode(value: 3, children: [node5, node6])
let node2 = TreeNode(value: 2, children: [node4])
let node1 = TreeNode(value: 1, children: [node2,node3])
let node0 = TreeNode(value: 0, children: [node1])


func valueOfNodes(minNumberOfChildren: Int, node: TreeNode, tempResult: [Int])->[Int]{

    var result = tempResult

    guard let children = node.children else{
        return result
    }

    if children.count >= minNumberOfChildren{
        result.append(node.value)
    }

    for child in children{
        return valueOfNodes(minNumberOfChildren: minNumberOfChildren, node: child, tempResult: result)
    }

    return result
}


let result = valueOfNodes(minNumberOfChildren: 2, node: node0, tempResult: [])
print(result)

Answer 1

You always return result only for first child:

for child in children{
    return valueOfNodes(minNumberOfChildren: minNumberOfChildren, node: child, tempResult: result)
}

Instead you need to combine them

let childrenResult = children.flatMap { child in
    return valueOfNodes(minNumberOfChildren: minNumberOfChildren, node: child, tempResult: [])
}
result.append(contentsOf: childrenResult)

Such approach kills tail recursion optimization, but I don't think you can use it here, since result of parent depends on results of all children.