队列

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const int maxn = 1e5+5; //队列的最大容量
int que[maxn]; //队列的存储空间
int head= 0, tail = 0;  //队头和队尾

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void push (int n){
    que[tail++] = n; //为什么是tail++,不是++tail?
}

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void pop() {
    head++;
}

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int front() {
    return que[head];
}

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int back() {
    return que[tail-1];
}

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bool empty() {
    return head == tail;
}

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int size() {
    return tail-head;
}

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template<typename T = int,int siz = maxn>
struct myqueue{
    T a[siz+5];
    //tail 指向最后一个元素后面一个位置
    //head 指向第一个元素
    int head = 0,tail=0; 

    void clear() { head =tail = 0;}

    void push(T b) { a[tail++] = b;}

    void pop(){head++;}
    void pop_back(){tail--;}

    T front() { return a[head];}
    T back() { return a[tail-1];}

    bool empty() { return head == tail;}

    int size() { return tail-head;}
};