队列

队列

• 一种线性结构
• 相比数组,队列对应的操作是数组的子集
• 只能从一端(队尾)添加元素,另一端(队首)取出元素
• 是一种先进先出的数据结构(FIFO)
• 队列的实现

基于数组实现

public class ArrayQueue{
    
    private Array array;

    public ArrayQueue(int capacity){
        array = new Array(capacity);
    }

    public ArrayQueue(){
        array = new Array();
    }

    public int getSize(){
        return array.getSize();
    }

    public boolean isEmpty(){
        return array.isEmpty();
    }

    public int getCapacity(){
        return array.getCapacity();
    }

    public void enqueue(int e){
        array.addLast(e);
    }

    public int dequeue(){
        return array.removeFirst();
    }

    public int getFront(){
        return array.getFirst();
    }
}

基于链表实现

public class LinkedListQueue{
    private calss Node{
        public Integer e;

        public Node next;

        public Node(Integer e, Node next){
            this.e = e;
            this.next = next;
        }

        public Node(Integer e){
            this(e,null);
        }

        public Node(){
            this(null,null);
        }
    }

    private Node head;
    private Node tail;
    private int size;

    public LinkedListQueue(){
        head = null;
        tail = null;
        size = 0;
    }

    public int getSize(){
        return size;
    }

    public boolean isEmpty(){
        return size == 0;
    }

    public void enqueue(Integer e){
        if(tail == null){
            tail = new Node(e);
            head = tail
        }else{
            tail.next = new Node(e);
            tail = tail.next;
        }
        size++;
    }

    public Integer dequeue(){
        if(isEmpty()){
            throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
        }
        Node retNode = head;
        head = head.next;
        retNode.next = null;
        if(head == null){
            tail = null;
        }
        size--;
        return retNode.e;
    }
}

循环队列

• 循环队列的实现

public class LoopQueue{
    private int[] data;
    private int front;
    private int tail;
    private int size;

    public LoopQueue(int capacity){
        data = new int[capacity+1];
        front = 0;
        tail = 0;
        size = 0
    }

    public LoopQueue(){
        this(10);
    }

    public int getCapacity(){
        return data.length - 1;
    }

    public boolean isEmpty(){
        return front == tail;
    }

    public int getSize(){
        return size;
    }

    public void enqueue(int e){
        if((tail + 1)%data.length == front){
            resize(getCapacity() * 2);
        }
        data[tail] = e;
        tail = (tail + 1)%data.length;
        size++;
    }

    public int dequeue(){
        if(isEmpty()){
            throw new IllegalArgumentException("Cannot dequeue from an empty queue");
        }
        int ret = data[front];
        front = (front+1)%data.length;
        size--;
        if(size == getCapacity()/4 && getCapacity()/2 != 0){
            resize(getCapacity()/2);
        }
        return ret;
    }

    public int getFront(){
        if(isEmpty()){
            throw new IllegalArgumentException("Cannot dequeue from an empty queue");
        }
        return data[front];
    }

    private void resize(int newCapacity){
        int[] newData = new int[newCapacity+1];
        for(int i = 0; i < size; i++){
            newData[i] = data[(i + front)%data.length];
        }
        data = newData;
        front = 0;
        tail = size;
    }
}