Task: Tree traversal
C: #include <stdlib.h>
#include <stdio.h>

typedef struct node_s
{
  int value;
  struct node_s* left;
  struct node_s* right;
} *node;

node tree(int v, node l, node r)
{
  node n = malloc(sizeof(struct node_s));
  n->value = v;
  n->left  = l;
  n->right = r;
  return n;
}

void destroy_tree(node n)
{
  if (n->left)
    destroy_tree(n->left);
  if (n->right)
    destroy_tree(n->right);
  free(n);
}

void preorder(node n, void (*f)(int))
{
  f(n->value);
  if (n->left)
    preorder(n->left, f);
  if (n->right)
    preorder(n->right, f);
}

void inorder(node n, void (*f)(int))
{
  if (n->left)
    inorder(n->left, f);
  f(n->value);
  if (n->right)
    inorder(n->right, f);
}

void postorder(node n, void (*f)(int))
{
  if (n->left)
    postorder(n->left, f);
  if (n->right)
    postorder(n->right, f);
  f(n->value);
}


typedef struct qnode_s
{
  struct qnode_s* next;
  node value;
} *qnode;

typedef struct { qnode begin, end; } queue;

void enqueue(queue* q, node n)
{
  qnode node = malloc(sizeof(struct qnode_s));
  node->value = n;
  node->next = 0;
  if (q->end)
    q->end->next = node;
  else
    q->begin = node;
  q->end = node;
}

node dequeue(queue* q)
{
  node tmp = q->begin->value;
  qnode second = q->begin->next;
  free(q->begin);
  q->begin = second;
  if (!q->begin)
    q->end = 0;
  return tmp;
}

int queue_empty(queue* q)
{
  return !q->begin;
}

void levelorder(node n, void(*f)(int))
{
  queue nodequeue = {};
  enqueue(&nodequeue, n);
  while (!queue_empty(&nodequeue))
  {
    node next = dequeue(&nodequeue);
    f(next->value);
    if (next->left)
      enqueue(&nodequeue, next->left);
    if (next->right)
      enqueue(&nodequeue, next->right);
  }
}

void print(int n)
{
  printf("%d ", n);
}

int main()
{
  node n = tree(1,
                tree(2,
                     tree(4,
                          tree(7, 0, 0),
                          0),
                     tree(5, 0, 0)),
                tree(3,
                     tree(6,
                          tree(8, 0, 0),
                          tree(9, 0, 0)),
                     0));

  printf("preorder:    ");
  preorder(n, print);
  printf("\n");

  printf("inorder:     ");
  inorder(n, print);
  printf("\n");

  printf("postorder:   ");
  postorder(n, print);
  printf("\n");

  printf("level-order: ");
  levelorder(n, print);
  printf("\n");

  destroy_tree(n);

  return 0;
}

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);
		
	}
}