Task: Tree traversal
C#: using System;
using System.Collections.Generic;
using System.Linq;

class Node
{
    int Value;
    Node Left;
    Node Right;

    Node(int value = default(int), Node left = default(Node), Node right = default(Node))
    {
        Value = value;
        Left = left;
        Right = right;
    }

    IEnumerable<int> Preorder()
    {
        yield return Value;
        if (Left != null)
            foreach (var value in Left.Preorder())
                yield return value;
        if (Right != null)
            foreach (var value in Right.Preorder())
                yield return value;
    }

    IEnumerable<int> Inorder()
    {
        if (Left != null)
            foreach (var value in Left.Inorder())
                yield return value;
        yield return Value;
        if (Right != null)
            foreach (var value in Right.Inorder())
                yield return value;
    }

    IEnumerable<int> Postorder()
    {
        if (Left != null)
            foreach (var value in Left.Postorder())
                yield return value;
        if (Right != null)
            foreach (var value in Right.Postorder())
                yield return value;
        yield return Value;
    }

    IEnumerable<int> LevelOrder()
    {
        var queue = new Queue<Node>();
        queue.Enqueue(this);
        while (queue.Any())
        {
            var node = queue.Dequeue();
            yield return node.Value;
            if (node.Left != null)
                queue.Enqueue(node.Left);
            if (node.Right != null)
                queue.Enqueue(node.Right);
        }
    }

    static void Main()
    {
        var tree = new Node(1, new Node(2, new Node(4, new Node(7)), new Node(5)), new Node(3, new Node(6, new Node(8), new Node(9))));
        foreach (var traversal in new Func<IEnumerable<int>>[] { tree.Preorder, tree.Inorder, tree.Postorder, tree.LevelOrder })
            Console.WriteLine("{0}:\t{1}", traversal.Method.Name, string.Join(" ", traversal()));
    }
}

Go: package main

import "fmt"
    
type node struct {
    value       int
    left, right *node
}

func (n *node) iterPreorder(visit func(int)) {
    if n == nil {
        return
    }
    visit(n.value)
    n.left.iterPreorder(visit)
    n.right.iterPreorder(visit)
}

func (n *node) iterInorder(visit func(int)) {
    if n == nil {
        return
    }
    n.left.iterInorder(visit)
    visit(n.value)
    n.right.iterInorder(visit)
}

func (n *node) iterPostorder(visit func(int)) {
    if n == nil {
        return
    }
    n.left.iterPostorder(visit)
    n.right.iterPostorder(visit)
    visit(n.value)
}

func (n *node) iterLevelorder(visit func(int)) {
    if n == nil {
        return
    }
    for queue := []*node{n}; ; {
        n = queue[0]
        visit(n.value)
        copy(queue, queue[1:])
        queue = queue[:len(queue)-1]
        if n.left != nil {
            queue = append(queue, n.left)
        }
        if n.right != nil {
            queue = append(queue, n.right)
        }
        if len(queue) == 0 {
            return
        }
    }
}

func main() {
    tree := &node{1,
        &node{2,
            &node{4,
                &node{7, nil, nil},
                nil},
            &node{5, nil, nil}},
        &node{3,
            &node{6,
                &node{8, nil, nil},
                &node{9, nil, nil}},
            nil}}
    fmt.Print("preorder:    ")
    tree.iterPreorder(visitor)
    fmt.Println()
    fmt.Print("inorder:     ") 
    tree.iterInorder(visitor)
    fmt.Println()
    fmt.Print("postorder:   ")
    tree.iterPostorder(visitor)
    fmt.Println() 
    fmt.Print("level-order: ")
    tree.iterLevelorder(visitor)
    fmt.Println()
}

func visitor(value int) {
    fmt.Print(value, " ")
}