Program to implement simple right in-threaded binary trees

#include <stdio.h>
#include <conio.h>
struct tree
{
int val;
struct tree *right;
struct tree *left;
int thread;
};
struct tree *root = NULL;
struct tree* insert_node(struct tree *root, struct tree *ptr, struct tree *rt)
{
if(root == NULL)
{
root = ptr;
if(rt != NULL)
 {
root–>right = rt;
root–>thread = 1;
 }
}
else if(ptr–>val < root–>val)
root–>left = insert_node(root–>left, ptr, root);
else
if(root–>thread == 1)
 {
root–>right = insert_node(NULL, ptr, rt);
root–>thread=0;
 }
else
root–>right = insert_node(root–>right, ptr, rt);
return root;
}
struct tree* create_threaded_tree()
{
struct tree *ptr;
int num;
printf("\n Enter the elements, press –1 to terminate ");
scanf("%d", &num);
while(num != –1)
{
ptr = (struct tree*)malloc(sizeof(struct tree));
ptr–>val = num;
ptr–>left = ptr–>right = NULL;
ptr–>thread = 0;
root = insert_node(root, ptr, NULL);
printf(" \n Enter the next element ");
fflush(stdin);
scanf("%d", &num);
}
return root;
}
void inorder(struct tree *root)
{
struct tree *ptr = root, *prev;
do
{
while(ptr != NULL)
 {
prev = ptr;
ptr = ptr–>left;


if(prev != NULL)
 {
printf(" %d", prev–>val);
ptr = prev–>right;
while(prev != NULL && prev–>thread)
 {
printf(" %d", ptr–>val);
prev = ptr;
ptr = ptr–>right;
 }
 }
}while(ptr != NULL);
}
void main()
{
 // struct tree *root=NULL;
clrscr();
create_threaded_tree();
printf(" \n The in–order traversal of the tree can be given as : ");
inorder(root);
getch();
}