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()));
    }
}

Python: from collections import namedtuple
 
Node = namedtuple('Node', 'data, left, right')
tree = Node(1,
            Node(2,
                 Node(4,
                      Node(7, None, None),
                      None),
                 Node(5, None, None)),
            Node(3,
                 Node(6,
                      Node(8, None, None),
                      Node(9, None, None)),
                 None))
 
def printwithspace(i):
    print(i, end=' ')

def dfs(order, node, visitor):
    if node is not None:
        for action in order:
            if action == 'N':
                visitor(node.data)
            elif action == 'L':
                dfs(order, node.left, visitor)
            elif action == 'R':
                dfs(order, node.right, visitor)
                
def preorder(node, visitor = printwithspace):
    dfs('NLR', node, visitor)
 
def inorder(node, visitor = printwithspace):
    dfs('LNR', node, visitor)
 
def postorder(node, visitor = printwithspace):
    dfs('LRN', node, visitor)
 
def ls(node, more, visitor, order='TB'):
    "Level-based Top-to-Bottom or Bottom-to-Top tree search"
    if node:
        if more is None:
            more = []
        more += [node.left, node.right]
    for action in order:
        if action == 'B' and more:
            ls(more[0], more[1:], visitor, order)
        elif action == 'T' and node:
            visitor(node.data)

def levelorder(node, more=None, visitor = printwithspace):
    ls(node, more, visitor, 'TB') 
 

def reverse_preorder(node, visitor = printwithspace):
    dfs('RLN', node, visitor)
    
def bottom_up_order(node, more=None, visitor = printwithspace, order='BT'):
    ls(node, more, visitor, 'BT')


if __name__ == '__main__':
    w = 10
    for traversal in [preorder, inorder, postorder, levelorder, 
                      reverse_preorder, bottom_up_order]:
        if traversal == reverse_preorder:
            w = 20
            print('\nThe generalisation of function dfs allows:')
        if traversal == bottom_up_order:
            print('The generalisation of function ls allows:')
        print(f"{traversal.__name__:>{w}}:", end=' ')
        traversal(tree)
        print()