stack and queue using linked list

#include<bits/stdc++.h>

using namespace std;

struct Node{
    int data;
    Node *link;
};

Node *top = NULL;
Node *fronts = NULL;
Node *rear = NULL;

bool isEmpty(){
    if(top == NULL){
        return true;
    }else{
        return false;
    }
}

bool qisEmpty(){
    if(fronts == NULL && rear == NULL){
        return true;
    }else{
        return false;
    }
}

void enqueue(int value){
    Node *p = new Node();
    p->data = value;
    p->link = NULL;

    if(fronts == NULL){
        fronts = p;
        rear = p;
    }else{
        rear->link = p;
        rear = p;
    }
}

void dequeue(){
    if(qisEmpty()){
        cout << "Queue is Empty!";
    }else{
        if(fronts == rear){
            free(fronts);
            fronts = rear = NULL;
        }else{
            Node *p = fronts;
            fronts = fronts->link;
            free(p);
        }
    }
}

void showfront(){
    if(qisEmpty()){
        cout << "Queue is Empty!";
    }else{
        cout << fronts->data;
    }

}

void displayQueue(){
    if(qisEmpty()){
        cout << "Queue is Empty!";
    }else{
        Node *p = fronts;
        while(p != NULL){
            cout << p->data << " ";
            p = p->link;
        }
    }
}

void push(int value){
    Node *p = new Node();
    p->data = value;
    p->link =top;
    top = p;
}

void pop(){
    if(isEmpty()){
        cout << "Stack is Empty!";
    }else{
        Node *p = top;
        top = top->link;
        delete(p);
    }
}

void showTop(){
    if(isEmpty()){
        cout << "Stack is Empty!";
    }else{
        cout << "Element at top is: " << top->data;
    }
}

int main(){
    //Stack
    push(1);
    push(2);
    pop();
    push(5);
    showTop();

    //Queue
    int choice, flag=1, value;
    while(flag==1){
        cout<<"\n1.enqueue 2.dequeue 3.showfront 4.displayQueue 5.exit\n";
        cin>>choice;
        switch (choice){
            case 1: cout<<"Enter Value:\n";
              cin>>value;
              enqueue(value);
              break;
            case 2: dequeue();
              break;
            case 3: showfront();
              break;
            case 4: displayQueue();
              break;
            case 5: flag = 0;
              break;
        }
    }

    return 0;
}