#include<iostream>

using namespace std;

struct node
{
    int value;
    struct node *next;
};
struct node *head = NULL;
struct node *tail = NULL;

void insertHead(int val)
{
    //create a new node
    struct node *newItem;
    newItem=(struct node *)malloc(sizeof(struct node));
    newItem->value = val;
    newItem->next = NULL;

    if (head == NULL) tail = newItem;
    //insert the new node at the head
    newItem->next = head;
    head = newItem;
}
void insertTail(int val)
{
    //create a new node to be inserted
    struct node *newItem;
    newItem=(struct node *)malloc(sizeof(struct node));
    newItem->value = val;
    newItem->next = NULL;
    if (head == NULL)
    {
        head = newItem;
        tail = newItem;
        return;
    }

    newItem->next = NULL;
    tail->next = newItem;
    tail = newItem;
}
void printList()
{
    if (head == NULL) return;
    struct node *cur =  head;
    while (cur != NULL)
    {
        printf("%d \t", cur->value);
        cur = cur->next;
    }
}
int deleteHead()
{
    struct node *cur;
    if (head == NULL) return -1;
    cur = head;
    head = head->next;

    if (head == NULL) tail = NULL;

    int val = cur->value;
    free(cur);
    return val;
}

class QUEUE{
    node* front;
    node* rear;

public:
    QUEUE(){
        front = NULL;
        rear = NULL;
    }

    void push_back(int val){
        //create a new node to be inserted
        struct node *newItem;
        newItem=(struct node *)malloc(sizeof(struct node));
        newItem->value = val;
        newItem->next = NULL;
        if (front == NULL)
        {
            front = newItem;
            rear = newItem;
            return;
        }

        newItem->next = NULL;
        rear->next = newItem;
        rear = newItem;
    }

    int pop(){
        struct node *cur;
        if (front == NULL) return -1;
        cur = front;
        front = front->next;

        if (front == NULL) rear = NULL;

        int val = cur->value;
        free(cur);
        return val;
    }
};


int main(){
    QUEUE q;
    q.push_back(10);
    q.push_back(20);
    q.push_back(30);

    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
    cout<<q.pop()<<endl;
}