GeeksforGeeks - Implement Stack and Queue using Deque - GoldmanSachs - Easy

struct DQueNode {
    int value;
    DQueNode* next;
    DQueNode* prev;
};
class Deque {
private:
    DQueNode* head;
    DQueNode* tail;
public:
    Deque(){
        head = tail = NULL;
    }
    bool isEmpty(){
        if (head == NULL)
            return true;
        return false;
    }
    int size(){
        if (!isEmpty()) {
            DQueNode* temp = head;
            int len = 0;
            while (temp != NULL) {
                len++;
                temp = temp->next;
            }
            return len;
        }
        return 0;
    }
    void insert_first(int element){
        DQueNode* temp = new DQueNode[sizeof(DQueNode)];
        temp->value = element;
        if (head == NULL) {
            head = tail = temp;
            temp->next = temp->prev = NULL;
        }
        else {
            head->prev = temp;
            temp->next = head;
            temp->prev = NULL;
            head = temp;
        }
    }
    void insert_last(int element){
        DQueNode* temp = new DQueNode[sizeof(DQueNode)];
        temp->value = element;
        if (head == NULL) {
            head = tail = temp;
            temp->next = temp->prev = NULL;
        }
        else {
            tail->next = temp;
            temp->next = NULL;
            temp->prev = tail;
            tail = temp;
        }
    }
     void remove_first(){
        if (!isEmpty()) {
            DQueNode* temp = head;
            head = head->next;
            if(head) head->prev = NULL;
            delete temp;
            if(head == NULL) tail = NULL;
            return;
        }
        cout << "List is Empty" << endl;
    }
    void remove_last(){
        if (!isEmpty()) {
            DQueNode* temp = tail;
            tail = tail->prev;
            if(tail) tail->next = NULL;
            delete temp;
            if(tail == NULL) head = NULL;
            return;
        }
        cout << "List is Empty" << endl;
    }
    void display(){
        if (!isEmpty()) {
            DQueNode* temp = head;
            while (temp != NULL) {
                cout << temp->value << " ";
                temp = temp->next;
            }
            cout << endl;
            return;
        }
        cout << "List is Empty" << endl;
    }
};