Queue Using Single Linked List

#include<stdio.h>
#include<Windows.h>
#include<conio.h>

void menu();
void enque();
void deque();
void view();

char pilih;

struct data{
    int angka;
    struct data *next;
}*head=0,*curr,*tail=0;

int main(){
    menu();
}

void menu(){
    system("cls");
    printf("Main MENU\n===========================\n");
    printf("1. Enque\n2. Deque\n3. Display\n4. Exit\nMasukan Input : ");
    pilih=_getch();fflush stdin;
    switch(pilih){
    case '1':
        enque();
        break;
    case '2':
        deque();
        break;
    case '3':
        view();
        break;
    case '4':
        break;
    default :
        menu();
        break;
    }
}

void enque(){
    system("cls");
    curr=(struct data*)malloc(sizeof(data));
    printf("Masukan Angka : ");
    scanf("%d",&curr->angka);fflush stdin;
    if (head==0){
        curr->next=0;
        head=tail=curr;
    }
    else {
        tail->next=curr;
        curr->next=0;
        tail=curr;
    }
    printf("Data Berhasil Di enque");
    Sleep(500);
    menu();
}

void view(){
    system("cls");
    curr=head;
    if (head!=0){
        while (curr!=0){
            printf("%d",curr->angka);
            if(curr->next!=0)
                printf(" -> ");
            curr=curr->next;
        }
    }else
        printf("Tidak ada data yang dapat ditampilkan");
    getchar();
    menu();
}

void deque(){
    system("cls");
    if(head!=0){
        curr=head;
        head=curr->next;
        free(curr);
        printf("Data Pertama Berhasil dihapus");
    }
    else
        printf("Tidak ada data yang bisa di deque");
    Sleep(500);
    menu();
}