【数据结构】算法 Design Front Middle Back Queue 设计前中后队列

目录

• Design Front Middle Back Queue 设计前中后队列
  • Description
  • 思路

Design Front Middle Back Queue 设计前中后队列

Description

Design a queue that supports push and pop operations in the front, middle, and back.

Implement the FrontMiddleBack class:

• FrontMiddleBack() Initializes the queue.
• void pushFront(int val) Adds val to the front of the queue.
• void pushMiddle(int val) Adds val to the middle of the queue.
• void pushBack(int val) Adds val to the back of the queue.
• int popFront() Removes the front element of the queue and returns it. If the queue is empty, return -1.
• int popMiddle() Removes the middle element of the queue and returns it. If the queue is empty, return -1.
• int popBack() Removes the back element of the queue and returns it. If the queue is empty, return -1.

Notice that when there are two middle position choices, the operation is performed on the frontmost middle position choice. For example:

• Pushing 6 into the middle of [1, 2, 3, 4, 5] results in [1, 2, 6, 3, 4, 5].
• Popping the middle from [1, 2, 3, 4, 5, 6] returns 3 and results in [1, 2, 4, 5, 6].

思路

通过2个双端队列拼接，实现一个队列的前中后插入删除

class FrontMiddleBackQueue {
    
public class Node {
    int val;
    Node next;
    Node pre;

    public Node(int val) {
        this.val = val;
        this.next = null;
        this.pre  = null;
    }

    public void insert_pre(Node p){
        p.pre = pre;
        p.next = this;
        if (this.pre!=null){
            this.pre.next =p;
        }
        this.pre =p;
        return;
    }

    public void insert_next(Node p){
        p.next = this.next;
        p.pre = this;
        if (this.next!=null){
            this.next.pre = p;
        }
        this.next =p;
        return;
    }

    public void del_pre(){
        if(this.pre ==null){
            return;
        }
        Node p = this.pre;
        this.pre = p.pre;
        if (p.pre!=null){
            p.pre.next = this;
        }
        return;
    }

    public void del_next(){
        if(this.next ==null){
            return;
        }
        Node p = this.next;
        this.next = p.next;
        if (p.next!=null){
            p.next.pre = this;
        }
        return;
    }
}

 public class Queue {
        public Node head;//头节点,head中不存数据
        public Node tail;//尾节点后一位，tail不存数据
        public int cnt;//队列中元素个数

    Queue(){
        cnt = 0;
        tail = new Node(0);
        head = new Node(0);
        head.next = tail;
        head.pre =null;
        tail.next = null;
        tail.pre = head;
    }

    public int size(){
        return cnt;
    }

    public boolean isEmpty(){
        return head.next == tail;
    }

    //尾部入队 ，tail前插
    void push_back(int val){
        tail.insert_pre(new Node(val));
        cnt+=1;
    }
    //头部入队，head后插
    void push_front(int val){
        head.insert_next(new Node(val));
        cnt+=1;
    }

    //尾部出队，tail前删
    int pop_back(){
        if (isEmpty()){
            return  -1;
        }
        int ret = tail.pre.val;
        tail.del_pre();
        cnt-=1;
        return ret;
    }
    //头部出队，head后删
    int pop_front(){
        if (isEmpty()){
            return  -1;
        }
        int ret = head.next.val;
        head.del_next();
        cnt-=1;
        return ret;
    }

    //获取队首值
    int front(){
        return head.next.val;
    }
    int back(){
        return tail.pre.val;
    }
}
////////////////////////

    public Queue q1;
    public Queue q2;
    public FrontMiddleBackQueue() {
        q1 = new Queue();
        q2 = new Queue();
    }
    
    

    boolean isEmpty(){
        return q1.size() ==0;
    }

    //调整队列平衡  一般q1比q2多一个
    void update(){
        if (q1.size()<q2.size()){
            q1.push_back(q2.front());
            q2.pop_front();
        }
        if(q1.size()==q2.size()+2){
            q2.push_front(q1.back());
            q1.pop_back();
        }

    }
    //队列前插
    void pushFront(int val){
        q1.push_front(val);
        update();
    }
    //队列中插，在调整后插入q1尾
    void pushMiddle(int val){
        if (q1.size()>q2.size()){
            q2.push_front(q1.back());
            q1.pop_back();
        }
        q1.push_back(val);
    }
    //队列尾插
    void pushBack(int val){
        q2.push_back(val);
        update();
    }
    //队首出队
    int popFront(){
        if (isEmpty()){
            return  -1;
        }
        int ret = q1.pop_front();
        update();
        return ret;
    }
    //队中出队，q1尾出，update
    int popMiddle(){
        if (isEmpty()){
            return  -1;
        }
        int ret = q1.pop_back();
        update();;
        return ret;
    }
    //队尾出队，一般q2尾出，也有可能q1 尾出
    int popBack(){
        if (isEmpty()){
            return  -1;
        }
        int ret;
        if (!q2.isEmpty()){
            ret =q2.pop_back();
        }
        else {
            ret = q1.pop_back();
        }
        update();
        return ret;
    }
}

/**
 * Your FrontMiddleBackQueue object will be instantiated and called as such:
 * FrontMiddleBackQueue obj = new FrontMiddleBackQueue();
 * obj.pushFront(val);
 * obj.pushMiddle(val);
 * obj.pushBack(val);
 * int param_4 = obj.popFront();
 * int param_5 = obj.popMiddle();
 * int param_6 = obj.popBack();
 */