안녕하십니까 저는 오늘 처음 가입한 change1995입니다

#include <stdio.h>
#include <stdlib.h>
typedef int element;
struct ListNode {
	struct ListNode * link;
	element data;
};
typedef struct ListNode ListNode;
 
void error(char *message) {
	fprintf(stderr, "%s\n", message);
	exit(1);
}
void insert(ListNode ** phead, ListNode * p, ListNode * new_node)
{
	if (*phead == NULL) {
		new_node->link = NULL;
		*phead = new_node;
	}
	else if (p == NULL) {
		new_node->link = *phead;
		*phead = new_node;
	}
	else {
		new_node->link = p->link;
		p->link = new_node;
	}
}
 
void remove_node(ListNode ** phead, ListNode *p, ListNode * removed)
{
	if (p == NULL) {
		*phead = (*phead)->link;
	}
	else
		p->link = removed->link;
	free(removed);
}
 
void display(ListNode *head) {
	ListNode *p = head;
	while (p != NULL) {
		printf("%d->", p->data);
		p = p->link;
	}
	printf("\n");
}
void display_recur(ListNode *head) {
	ListNode *p = head;
	while (p != NULL) {
		printf("%d->", p->data);
		display_recur(p->link);
	}
}
 
ListNode *search(ListNode *head, int x) {
	ListNode *p;
	p = head;
	while (p != NULL) {
		if (p->data == x) return p;
	}
	return p;
}
 
ListNode *concat(ListNode * head1, ListNode * head2) {
	ListNode *p;
	if (head1 == NULL) return head2;
	else if (head2 == NULL) return head1;
	else {
		p = head1;
		while (p->link != NULL) p = p->link;
		p->link = head2;
		return head1;
	}
}
ListNode * reverse(ListNode * head) {
	ListNode * p, *q, *r;
	p = head;
	q = NULL;
	while (p != NULL) {
		r = q;
		q = p;
		p = p->link;
		q->link = r;
	}
	return q;
}
 
ListNode *creat_node(element data, ListNode *link) {
	ListNode *new_node;
	new_node = (ListNode*)malloc(sizeof(ListNode));
	if (new_node == NULL) error("메모리 할당 에러");
	new_node->data = data;
	new_node->data = link;
	return(new_node);
}
 
void main() {
	ListNode * list1 = NULL, *list2 = NULL;
	ListNode *p;
 
	insert(&list1, NULL, creat_node(10, NULL));
	insert(&list1, NULL, creat_node(20, NULL));
	insert(&list1, NULL, creat_node(30, NULL));
	display(list1);
 
	remove_node(&list1, NULL, list1);
	display(list1);
 
	insert(&list2, NULL, creat_node(60, NULL));
	insert(&list2, NULL, creat_node(70, NULL));
	insert(&list2, NULL, creat_node(80, NULL));
	display(list2);
 
	list1 = concat(list1, list2);
	display(list1);
	list1 = reverse(list1);
	display(list1);
 
	p = search(list1, 20);
	printf("탐색성공 : %d\n", p->data);
}