struct Tree {
  int val;
  Tree left;
  Tree right;
  (int) -> void f
}

void print(int val) {
	print_string(string_cat("->", string_of_int(val)));
	return;
}

void in_order_traverse(Tree tree) {
	tree.f(tree.val);
	if(tree.left.val != 0) {
		in_order_traverse(tree.left);
	}

	if(tree.right.val != 0) {
		in_order_traverse(tree.right);
	}
	return;
}

void pre_order_traverse(Tree tree) {
	if(tree.left.val != 0) {
		pre_order_traverse(tree.left);
	}

	tree.f(tree.val);

	if(tree.right.val != 0) {
		pre_order_traverse(tree.right);
	}
	return;
}

void post_order_traverse(Tree tree) {
	if(tree.left.val != 0) {
		post_order_traverse(tree.left);
	}

	if(tree.right.val != 0) {
		post_order_traverse(tree.right);
	}

	tree.f(tree.val);
	return;
}

int program(int argc, string[] argv) {
	var nullchild = new Tree { val = 0; left = Tree null; right = Tree null; f = print};
	var rightchild = new Tree { val = 1; left = nullchild; right = nullchild; f = print }; 
	var leftchild = new Tree { val = 1; left = nullchild; right = nullchild; f = print }; 
	var tree = new Tree { val = 2; left = leftchild; right = rightchild; f = print };

	pre_order_traverse(tree);
	in_order_traverse(tree);
	post_order_traverse(tree);
	return 0;
}