Task: Tree traversal
Java: import java.util.*;

public class TreeTraversal {

        static class Node<T> {
		T value;
		Node<T> left;
		Node<T> right;

		Node(T value) {
			this.value = value;
		}

		void visit() {
			System.out.print(this.value + " ");
		}
	}

	static enum ORDER {
		PREORDER, INORDER, POSTORDER, LEVEL
	}
        
        static <T> void traverse(Node<T> node, ORDER order) {
		if (node == null) {
			return;
		}
		switch (order) {
		case PREORDER:
			node.visit();
			traverse(node.left, order);
			traverse(node.right, order);
			break;
		case INORDER:
			traverse(node.left, order);
			node.visit();
			traverse(node.right, order);
			break;
		case POSTORDER:
			traverse(node.left, order);
			traverse(node.right, order);
			node.visit();
			break;
		case LEVEL:
			Queue<Node<T>> queue = new LinkedList<>();
			queue.add(node);
			while(!queue.isEmpty()){
				Node<T> next = queue.remove();
				next.visit();
				if(next.left!=null)
					queue.add(next.left);
				if(next.right!=null)
					queue.add(next.right);
			}
		}
	}	

	public static void main(String[] args) {

		Node<Integer> one = new Node<Integer>(1);
		Node<Integer> two = new Node<Integer>(2);
		Node<Integer> three = new Node<Integer>(3);
		Node<Integer> four = new Node<Integer>(4);
		Node<Integer> five = new Node<Integer>(5);
		Node<Integer> six = new Node<Integer>(6);
		Node<Integer> seven = new Node<Integer>(7);
		Node<Integer> eight = new Node<Integer>(8);
		Node<Integer> nine = new Node<Integer>(9);
		
		one.left = two;
		one.right = three;
		two.left = four;
		two.right = five;
		three.left = six;
		four.left = seven;
		six.left = eight;
		six.right = nine;

		traverse(one, ORDER.PREORDER);
		System.out.println(); 
		traverse(one, ORDER.INORDER);
		System.out.println();
		traverse(one, ORDER.POSTORDER);
		System.out.println();
		traverse(one, ORDER.LEVEL);
		
	}
}

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()