//trees.h

#define ALREADY_IN_TREE 1
#define OUT_OF_MEMORY 2

struct Tree {
  int elem;
  struct Tree *left;
  struct Tree *right;
};

extern struct Tree *insertElem (int, struct Tree *, int *);

// trees.c
#include <stdlib.h>
#include <trees.h>

/* insert element into tree; return error if already present */

struct Tree *insertElem (int elem, struct Tree *tree, int *errno) {
  if (tree != NULL) {
    if (elem == tree->elem) {
      *errno = ALREADY_IN_TREE;
      return (NULL);
    }

    if (elem < tree->elem) 
      tree->left = insertElem (elem, tree->left, errno);
    else 
      tree->right = insertElem (elem, tree->right, errno);
  }
  else {
    /* create a new leaf */
    tree = malloc (sizeof (struct Tree));
    if (!tree) {
      *errno = OUT_OF_MEMORY;
      return (NULL);
    }
    
    tree->elem = elem;
    tree->left = NULL;
    tree->right = NULL;
  }
  return tree;
}