C code to implement Threaded Binary Search Tree

/* C program to implement Threaded binary tree */

# include <stdio.h>
# include <malloc.h>
#define infinity 9999

typedef enum { thread,link} boolean;
struct node *in_succ(struct node *p);
struct node *in_pred(struct node *p);

struct node
{
    struct node *left_ptr;
    boolean left;
    int info;
    boolean right;
    struct node *right_ptr;
}*head=NULL;

int main()
{
    int choice,num;
    insert_head();
    while(1)
    {
        printf("\n");
        printf("1.Insert\n");
        printf("2.Inorder Traversal\n");
        printf("3.Quit\n");
        printf("Enter your choice : ");
        scanf("%d",&choice);

        switch(choice)
        {
        case 1:
            printf("Enter the number to be inserted : ");
            scanf("%d",&num);
            insert(num);
            break;
        case 2:
            inorder();
            break;
        case 3:
            exit();
        default:
            printf("Wrong choice\n");
        }/*End of switch */
    }/*End of while */
}/*End of main()*/

int insert_head()
{
    struct node *tmp;
    head=(struct node *)malloc(sizeof(struct node));
    head->info= infinity;
    head->left=thread;
    head->left_ptr=head;
    head->right=link;
    head->right_ptr=head;
}/*End of insert_head()*/

int find(int item,struct node **par,struct node **loc)
{
    struct node *ptr,*ptrsave;
    if(head->left_ptr==head) /* If tree is empty*/
    {
        *loc=NULL;
        *par=head;
        return;
    }
    if(item==head->left_ptr->info) /* item is at head->left_ptr */
    {
        *loc=head->left_ptr;
        *par=head;
        return;
    }
    ptr=head->left_ptr;
    while(ptr!=head)
    {
        ptrsave=ptr;
        if( item < ptr->info )
        {
            if(ptr->left==link)
                ptr=ptr->left_ptr;
            else
                break;
        }
        else
             if(item > ptr->info )
             {
            if(ptr->right==link)
                ptr=ptr->right_ptr;
            else
                break;
             }
        if(item==ptr->info)
        {
            *loc=ptr;
            *par=ptrsave;
            return;
        }
    }/*End of while*/
    *loc=NULL;   /*item not found*/
    *par=ptrsave;
}/*End of find()*/

/* Creating threaded binary search tree */

int insert(int item)
{
    struct node *tmp,*parent,*location;
    find(item,&parent,&location);

    if(location!=NULL)
    {
        printf("Item already present");
        return;
    }

    tmp=(struct node *)malloc(sizeof(struct node));
    tmp->info=item;
    tmp->left=thread;
    tmp->right=thread;

    if(parent==head) /*tree is empty*/
    {
        head->left=link;
        head->left_ptr=tmp;
        tmp->left_ptr=head;
        tmp->right_ptr=head;
    }
    else
        if( item < parent->info )
        {
            tmp->left_ptr=parent->left_ptr;
            tmp->right_ptr=parent;
            parent->left=link;
            parent->left_ptr=tmp;
        }
        else
        {
            tmp->left_ptr=parent;
            tmp->right_ptr=parent->right_ptr;
            parent->right=link;
            parent->right_ptr=tmp;
        }
}/*End of insert()*/

/* Finding succeeder */

struct node *in_succ(struct node *ptr)
{
    struct node *succ;
    if(ptr->right==thread)
        succ=ptr->right_ptr;
    else
    {
        ptr=ptr->right_ptr;
        while(ptr->left==link)
            ptr=ptr->left_ptr;
        succ=ptr;
    }
    return succ;
}/*End of in_succ()*/

/* Finding predecessor */

struct node *in_pred(struct node *ptr)
{
    struct node *pred;
    if(ptr->left==thread)
        pred=ptr->left_ptr;
    else
    {
        ptr=ptr->left_ptr;
        while(ptr->right==link)
            ptr=ptr->right_ptr;
        pred=ptr;
    }
    return pred;
}/*End of in_pred()*/

/* Displaying all nodes */

inorder()
{
    struct node *ptr;
    if(head->left_ptr==head)
    {
        printf("Tree is empty");
        return;
    }

    ptr=head->left_ptr;

    /*Find the leftmost node and traverse it */

    while(ptr->left==link)
        ptr=ptr->left_ptr;
    printf("%d ",ptr->info);

    while( 1 )
    {
        ptr=in_succ(ptr);
        if(ptr==head)     /*If last node reached */
            break;
        printf("%d ",ptr->info);
    } /*End of while*/
}/*End of inorder()*/

/* Output of Threaded Binary Search Tree Program */