奇奇怪怪的代码求助

本来想手动写一个双端队列玩玩，然后用几个题目测试一下，结果过不去。

using namespace std;
template <typename new_dype>
class Deque{
    struct De_node{
        new_dype val;
        De_node *next = NULL, *prev = NULL;
    }*tail, *head;
public:
    Deque():tail(NULL), head(NULL){
        return;
    }
    void pop_back(){
        if(tail){
            De_node *temp = tail;
            tail = tail->prev;
            if(tail)
                tail->next = NULL;
            delete temp;
        }
    }
    void pop_front(){
        if(head){
            De_node *temp = head;
            head = head->next;
            if(head)
                head->prev = NULL;
            delete temp;
        }
    }
    void push_back(new_dype num){
        De_node *newt = new De_node();
        newt->val = num;
        if(tail){
            tail->next = newt;
            newt->prev = tail;
        }else{
            newt->prev = NULL;
        }
        if(!head) head = newt;
        tail = newt;
    }
    void push_front(new_dype num){
        De_node *newt = new De_node();
        newt->val = num;
        if(head){
            newt->next = head;
            head->prev = newt;
        }else{
            newt->next = NULL;
        }
        if(!tail) tail = newt;
        head = newt;
    }
    new_dype front(){
        if(head)
            return head->val;
    }
    new_dype back(){
        if(tail)
            return tail->val;
    }
    void clear(){
        while(head){
            De_node *temp = head;
            head = head->next;
            delete temp;
        }
        tail = NULL;
    }
    bool empty(){
        return !(tail && head);
    }
};
int n, k;
struct node{
    int id, num;
}a[1000010];
Deque<node>q;
int main(){
    cin >> n >> k;
    for(int i = 1; i <= n; i++){
        cin >> a[i].num;
        a[i].id = i;
    }
    for(int i = 1; i <= n; i++){//最小值 
        while(!q.empty() && a[i].num < q.back().num){
            q.pop_back();
        }
        q.push_back(a[i]);
        while(i - q.front().id >= k){
            q.pop_front();
        }
        if(i >= k){
            cout << q.front().num << " ";
        }
    }
    q.clear();
    cout << endl;
    for(int i = 1; i <= n; i++){//最大值 
        while(!q.empty() && a[i].num > q.back().num){
            q.pop_back();
        }
        q.push_back(a[i]);
        while(i - q.front().id >= k){
            q.pop_front();
        }
        if(i >= k){
            cout << q.front().num << " ";
        }
    }
    return 0;
}