Creating and traversing a binary search tree

The need:

     This program is written to show creation and various types of printing of a binary search tree. This program takes values and put them in a binary search tree and then prints the resulting tree in preorder, inorder and post order.
 The BST(Binary Search tree): a binary search tree is a binary tree in which value of each node is greater or equal than that of its left child and less than that of its right child. Hence all nodes in the left sub-tree of a node are less than or equal to the value of that node and all nodes in the right sub-tree of a node are greater than the value of that node if it is a binary search tree. We'll use this property while inserting a node in the BST in insert() function.

The code:

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#define MAX 10
#define DEBUG if(!(root->left))    printf("inserted %d to the left of %d\n",x,root->data);
#include<stdio.h>
#include<stdlib.h>
struct tree_node
{
    int data;
    struct tree_node *left,*right,*parent;
};
typedef struct tree_node node;
node * new_node(int x);
node * insert_node(node *root, int x);
void print_preorder(node *root);
void print_inorder(node *root);
void print_postorder(node *root);

int main()
{
    node *root=NULL;
    int i;
    int arr[]={5,6,1,9,0,4,2,3,8,7};
    for(i=0;i<MAX;i++)
    root = insert_node(root, arr[i]);
    printf("\ntree pre order is\n");
    print_preorder(root);
    printf("\n\ntree in order is\n");
    print_inorder(root);
    printf("\n\ntree post order is\n");
    print_postorder(root);
    return 0;
}

node * insert_node(node *root, int x)
{
    if(!root)
    {
        root = new_node(x);
        return root;
    }
    if(root->data > x)
    {
//        DEBUG
        root->left = insert_node(root->left,x);
    }
    else
    {
//        DEBUG
        root->right = insert_node(root->right,x);
    }
    return root;
}

void print_preorder(node *root)
{
    if(!root)
    return;
    printf("%d ",root->data);
    print_preorder(root->left);
    print_preorder(root->right);
    return;
}

void print_inorder(node *root)
{
    if(!root)
    return;
    print_inorder(root->left);
    printf("%d ",root->data);
    print_inorder(root->right);
    return;
}

void print_postorder(node *root)
{
    if(!root)
    return;
    print_postorder(root->left);
    print_postorder(root->right);
    printf("%d ",root->data);
    return;
}

node * new_node(int x)
{
    node *furnished;
    furnished = (node*)malloc(sizeof(node));
    furnished->data=x;
    furnished->left=NULL;
    furnished->right=NULL;
    furnished->parent=NULL;
    return furnished;
}

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Approach:

    This program is just to show the printing of a tree in pre-order, inorder and post order. For that first we have to make the tree. Here we dont have tree with 3 nodes necessarily. So we need to make a function to construct the tree. Here insert() function does the job. create_node() function creates a node with given data/value ans returns it address. we create first node, make it root. After that we create 2 more nodes and make them left and right children of root. Then there are 3 functions in_print(), pre_print() and post_print() which print the expression in infix, prefix and postfix form respectively.

Remarks:

1. We noticed that here in each function I have used recursion like I have already said in previous posts that recursion makes our life easier in trees.
2. Uncomment DEBUG to see where the nodes are being inserted.
3. Here I am taking node data from an array we can take data from user also.