#include <stdio.h>
#include <stdlib.h>

typedef struct binary_tree{
  struct binary_tree *left;
  struct binary_tree *right;
  int value;
}binary_tree;

void init_btree(binary_tree *root);

// traverse the tree in pre-order recursively
void pre_order_r(const binary_tree *root){
  if(root == NULL){
    return;
  }
  printf("%d ", root->value);
  pre_order_r(root->left);
  pre_order_r(root->right);
}

int main() {
  binary_tree *root = (binary_tree*)malloc(sizeof(binary_tree*));;
  init_btree(root);
  pre_order_r(root);
  printf("n");
}

void init_btree(binary_tree *root){
  root->left = root->right = NULL;
  root->value = 1;
  binary_tree * p1 = (binary_tree*)malloc(sizeof(binary_tree*));
  p1->left = p1->right = NULL;
  p1->value = 2;
  binary_tree * p2 = (binary_tree*)malloc(sizeof(binary_tree*));
  p2->left = p2->right = NULL;
  p2->value = 3;

  root->left = p1;
  root->right = p2;
}