Two Stacks in one array.

// Implementation of two stacks in one array...

#include <iostream>
#include <stdlib.h>
using namespace std;

int SIZE, topA, topB, *arr, N1, N2;
void stacks_initiate() {
    topA = -1;
    topB = SIZE;
    arr = new int[SIZE];
}

void push_A(int);
void push_B(int);
int pop_A();
int pop_B();
int top_A();
int top_B();
void show_A();
void show_B();
void menu();

int main() {
    cout << "Stack size 01 : "; cin >> N1;
    cout << "Stack size 02 : "; cin >> N2;
    SIZE = N1 + N2;
    stacks_initiate();
    menu();
    return 0;
}

void push_A(int x) {
    if (topB == topA + 1)
        cout << "\nStack overflow. [ARRAY is full]" << endl;
    else if (topA == N1 - 1) {
        cout << "\nStack(A) overflow. It seems you can still push into the other stack from the menu.\n";
    }
    else {
        arr[++topA] = x;
        cout << "\nData " << x << " has been pushed into stack A." << endl;
    }
}

void push_B(int x) {
    if (topB == topA + 1)
        cout << "\nStack overflow. [ARRAY is full]" << endl;
    else if (topB == SIZE - N2) { // or simply top == N1 could be used..
        cout << "\nStack(B) overflow. It seems you can still push into the other stack from the menu.\n";
    }
    else {
        arr[--topB] = x;
        cout << "\nData " << x << " has been pushed into stack B." << endl;
    }
}

int pop_A() {
    if (topA == -1)
        cout << "\nStack underflow. [0 element in stack A]" << endl;
    else {
        int x = top_A();
        arr[topA--] = NULL;
        return x;
    }
}

int pop_B() {
    if (topB == SIZE)
        cout << "\nStack underflow. [0 element in stack B]" << endl;
    else {
        int x = top_B();
        arr[topB++] = NULL;
        return x;
    }
}

int top_A() {
    return arr[topA];
}

int top_B() {
    return arr[topB];
}

void show_A() {
    if (topA == -1) cout << "\nStack A is empty." << endl;
    else {
        cout << "\nStack A : ";
        for (int i = topA; i >= 0; i--)
        {
            cout << arr[i] << " ";
        }
        cout << endl;
    }
}

void show_B() {
    if (topB == SIZE) cout << "\nStack B is empty." << endl;
    else {
        cout << "\nStack B : ";
        for (int i = topB; i < SIZE; i++)
        {
            cout << arr[i] << " ";
        }
        cout << endl;
    }
}

void menu() {
    cout << "\n01. Push into A.\n02. Push into B.\n03. Pop from A.\n04. Pop from B.\n05. Get top from A.\n06. Get top from B.\n07. Show A.\n08. Show B.\n09. EXIT." << endl;
    cout << "\nChoice : "; int ch; int x;
    cin >> ch;
    switch (ch) {
    case 1: cout << "\nWhat do you want to push into A? Push : ";
        cin >> x;
        push_A(x); menu();
        break;
    case 2: cout << "\nWhat do you want to push into B? Push : ";
        cin >> x;
        push_B(x); menu();
        break;
    case 3: if (topA == -1) cout << "\nStack A is empty." << endl;
        else cout << "\nData " << pop_A() << " has been removed from stack A.\n"; menu();
        break;
    case 4: if (topB == SIZE) cout << "\nStack B is empty." << endl;
        else cout << "\nData " << pop_B() << " has been removed from stack B.\n"; menu();
        break;
    case 5: if (topA == -1) cout << "\nStack A is empty." << endl;
         else cout << "\nData at top of stack A : " << top_A() << endl; menu();
        break;
    case 6: if (topB == SIZE) cout << "\nStack B is empty." << endl;
        else cout << "\nData at top of stack B : " << top_B() << endl; menu();
        break;
    case 7: show_A(); menu();
        break;
    case 8: show_B(); menu();
        break;
    case 9: cout << "\nThank You..! Terminating Program...\n" << endl;
        delete []arr;
        exit(0);
        break;
    default: cout << "\nInvalid choice. Try again." << endl;
        menu();
    }
}