Task: Tree traversal
C++: #include <boost/scoped_ptr.hpp>
#include <iostream>
#include <queue>

template<typename T>
class TreeNode {
public:
  TreeNode(const T& n, TreeNode* left = NULL, TreeNode* right = NULL)
    : mValue(n),
      mLeft(left),
      mRight(right) {}

  T getValue() const {
    return mValue;
  }

  TreeNode* left() const {
    return mLeft.get();
  }

  TreeNode* right() const {
    return mRight.get();
  }

  void preorderTraverse() const {
    std::cout << " " << getValue();
    if(mLeft)  { mLeft->preorderTraverse();  }
    if(mRight) { mRight->preorderTraverse(); }
  }

  void inorderTraverse() const {
    if(mLeft)  { mLeft->inorderTraverse();  }
    std::cout << " " << getValue();
    if(mRight) { mRight->inorderTraverse(); }
  }

  void postorderTraverse() const {
    if(mLeft)  { mLeft->postorderTraverse();  }
    if(mRight) { mRight->postorderTraverse(); }
    std::cout << " " << getValue();
  }

  void levelorderTraverse() const {
    std::queue<const TreeNode*> q;
    q.push(this);

    while(!q.empty()) {
      const TreeNode* n = q.front();
      q.pop();
      std::cout << " " << n->getValue();

      if(n->left())  { q.push(n->left());  }
      if(n->right()) { q.push(n->right()); }
    }
  }

protected:
  T mValue;
  boost::scoped_ptr<TreeNode> mLeft;
  boost::scoped_ptr<TreeNode> mRight;

private:
  TreeNode();
};

int main() {
  TreeNode<int> root(1,
    new TreeNode<int>(2,
      new TreeNode<int>(4,
        new TreeNode<int>(7)),
      new TreeNode<int>(5)),
    new TreeNode<int>(3,
      new TreeNode<int>(6,
        new TreeNode<int>(8),
        new TreeNode<int>(9))));

  std::cout << "preorder:   ";
  root.preorderTraverse();
  std::cout << std::endl;

  std::cout << "inorder:    ";
  root.inorderTraverse();
  std::cout << std::endl;

  std::cout << "postorder:  ";
  root.postorderTraverse();
  std::cout << std::endl;

  std::cout << "level-order:";
  root.levelorderTraverse();
  std::cout << std::endl;

  return 0;
}

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()