LevelOrderTraversal.swift

extension TreeNode {
    public func levelOrderTraversal(visit: (TreeNode) -> Void) {
        visit(self)
        var queue = [TreeNode]()
        children.forEach { queue.append($0) }
        
        while !queue.isEmpty {
            let node = queue.removeFirst()
            visit(node)
            node.children.forEach { queue.append($0) }
        }
    }
}

let tree = createTree()
tree.levelOrderTraversal {
    print($0.value)
}