C++ 双端队列实现

#include <iostream>
using namespace std;
#define ulli size_t
template<class T>class DualStack {
private:
	struct Node {
		T data;
		Node* next;
	};
	Node* head, *tail;
	Node* p;
	ulli length;

public:
	DualStack() {
		head = NULL;
		length = 0;
		tail = NULL;
		p = NULL;
	}
	~DualStack() {
		Node* ptr = NULL;
		while (head != NULL) {
			ptr = head->next;
			delete head;
			head = ptr;
		}
	}
	void push_back(T n) {
		Node* q = new Node;
		q->data = n;
		if (head == NULL) {
			q->next = head;
			head = q;
			p = q;
		}
		else {
			q->next = p;
			p = q;
		}
		length++;
	}
	void push_front(T n) {
		Node* q = new Node;
		q->data = n;
		q->next = tail;
		tail = q;
		length++;
	}
	void pop_front() {
		if (length <= 0) {
			abort();
		}
		Node* q;
		T data;
		q = tail;
		data = tail->data;
		tail = tail->next;
		delete(q);
		length--;
	}
	void pop_back() {
		if (length <= 0) {
			abort();
		}
		Node* q;
		T data;
		q = head;
		data = head->data;
		head = head->next;
		delete(q);
		length--;
	}
	ulli size() {
		return length;
	}
	T front() {
		return tail->data;
	}
	T back() {
		return head->data;
	}
	bool empty() {
		if (length == 0) {
			return true;
		}
		else {
			return false;
		}
	}
	void clear() {
		while (length > 0) {
			pop_back();
			pop_front();
		}
	}
};