Threaded Binary Tree

#include <stdio.h>

enum pointer_tag { LINK, THREAD };

struct node {
  char* data;
  struct node* lchild;
  struct node* rchild;
  enum pointer_tag ltag, rtag;
};

void visit(char* data) {
  printf("%s ", data);
}

void traverse(struct node* head) {
  struct node* p = head->lchild;
  while (p != head) {
    while (p->ltag == LINK)
      p = p->lchild;
    visit(p->data);
    while (p->rtag == THREAD && p->rchild != head) {
      p = p->rchild;
      visit(p->data);
    }
    p = p->rchild;
  }
  printf("\n");
}

void set_node(struct node* node, char * data, struct node* lchild, struct node* rchild, enum 
    pointer_tag ltag, enum pointer_tag rtag) {
  node->data = data;
  node->lchild = lchild;
  node->rchild = rchild;
  node->ltag = ltag;
  node->rtag = rtag;
}

/*
 *      +
 *     / \
 *   a    b
 */
void test_1() {
  struct node root, node1, node2, node3;

  set_node(&root, "r", &node1, &node3, LINK, THREAD);

  set_node(&node1, "+", &node2, &node3, LINK, LINK);
  set_node(&node2, "a", &root, &node1, THREAD, THREAD);
  set_node(&node3, "b", &node1, &root, THREAD, THREAD);

  traverse(&root);
}
/*
 *           -
 *         /   \
 *        /     \
 *      +       "/"
 *    /   \    /    \
 *  a      *  e      f
 *       /   \
 *      b     -
 *          /   \
 *        c      d
 */
void test_2() {
  struct node root, node1, node2, node3, node4, node5, node6, node7, node8, 
              node9, node10, node11;

  set_node(&root, "r", &node1, &node7, LINK, THREAD);
  set_node(&node1, "-", &node2, &node3, LINK, LINK);
  set_node(&node2, "+", &node4, &node5, LINK, LINK);
  set_node(&node3, "/", &node6, &node7, LINK, LINK);

  set_node(&node4, "a", &root, &node2, THREAD, THREAD);
  set_node(&node5, "*", &node8, &node9, LINK, LINK);
  set_node(&node6, "e", &node1, &node3, THREAD, THREAD);
  set_node(&node7, "f", &node3, &root, THREAD, THREAD);

  set_node(&node8, "b", &node2, &node5, THREAD, THREAD);
  set_node(&node9, "-", &node10, &node11, LINK, LINK);
  set_node(&node10, "c", &node5, &node9, THREAD, THREAD);
  set_node(&node11, "d", &node9, &node1, THREAD, THREAD);

  traverse(&root);
}

int main(int argc, const char *argv[]) {
  test_1();
  test_2();
  return 0;
}