C program to reverse a Linked List

ll.c

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

struct node{
	int value;
	struct node *next;
};

struct node *HEAD = NULL;
struct node *temp, *y;
	
void insert(value){
	if(HEAD == NULL){
		temp = (struct node *)malloc(sizeof(struct node));
		HEAD = temp;
		temp->value = value;
		temp->next = NULL;
	}
	else{
		y = temp;
		temp = (struct node *)malloc(sizeof(struct node));
		y->next = temp;
		temp->value = value;
		temp->next = NULL;
	}
}

void display(){
	struct node *j;
	j = HEAD;
	while(j != NULL){
		printf("%d ",j->value);
		j = j->next;
	}
	printf("\n");
}

void reverse(){
	struct node *j, *next, *prev;
	j = HEAD;
	while(j != NULL){
		next = j->next;
		if(j==HEAD){
			j->next = NULL;
			prev = j;
		}
		else{
			j->next = prev;
			prev = j; 
		}
		j = next;
	}
	HEAD = prev;
	
}

int main(){
	insert(20);
	insert(30);

	insert(40);
	insert(23);

	display();

	reverse();
	
	display();

	return 0;
}