Java数据结构与算法-队列

参考《算法》（第四版）

一、队列的作用

队列是基于先进先出（FIFO）的集合。现实中类似的场景有在银行大厅排队、汽车过收费站排队等。队列的作用在于1）将数据保存在容器中，2）同时还保存了元素间相对顺序。

比如一个从文件中读取整数，并返回int数组的实现就可以用到队列，这样就不必提前知道文件大小：

public static int[] readInts(String name){
    In in = new In(Name);
    Queue<Integer> q = new Queue<Integer>();
    while(!in.isEmpty()) {
        q.enqueue(in.readInt());
    }
    
    int N = q.size();
    int[] a = new int[N];
    for(int i =0; i < N; i++) {
        a[i] = q.dequeue();
    }
    return a;
}

二、队列的实现

队列可以用1）链表 2）数组 来实现。

1、链表实现：

algs4中实现方法：

public class Queue<Item> implements Iterable<Item> {
    private Node<Item> first;    // 指向对首
    private Node<Item> last;     // 指向队尾
    private int n;               // 元素数量

    // 静态内部类存储元素，及链表指针
    private static class Node<Item> {
        private Item item;
        private Node<Item> next;
    }

    public Queue() {
        first = null;
        last  = null;
        n = 0;
    }

    public boolean isEmpty() {
        return first == null;
    }

    public int size() {
        return n;
    }

    // 返回对首
    public Item peek() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        return first.item;
    }

    //入队
    public void enqueue(Item item) {
        Node<Item> oldlast = last;
        last = new Node<Item>();
        last.item = item;
        last.next = null;
        if (isEmpty()) first = last;
        else           oldlast.next = last;
        n++;
    }
    //出队
    public Item dequeue() {
        if (isEmpty()) throw new NoSuchElementException("Queue underflow");
        Item item = first.item;
        first = first.next;
        n--;
        if (isEmpty()) last = null;   // to avoid loitering
        return item;
    }


    public Iterator<Item> iterator()  {
        return new ListIterator<Item>(first);  
    }

    
    private class ListIterator<Item> implements Iterator<Item> {
        private Node<Item> current;

        public ListIterator(Node<Item> first) {
            current = first;
        }

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }
}

 

2、数组实现：

参考ArrayQueue实现，利用环形数组，（数组长度取模）：

注意：

1）head，tail含义，head：队首；tail队尾可加入的空位。

2）有意保留一个空单元

public class ArrayQueue<T> extends AbstractList<T> {
    public ArrayQueue(int capacity) {
        this.capacity = capacity + 1;
        this.queue = newArray(capacity + 1);
        this.head = 0;
        this.tail = 0;
    }

    public void resize(int newcapacity) {
        int size = size();
        if (newcapacity < size)
            throw new IndexOutOfBoundsException("Resizing would lose data");
        newcapacity++;
        if (newcapacity == this.capacity)
            return;
        T[] newqueue = newArray(newcapacity);
        for (int i = 0; i < size; i++)
            newqueue[i] = get(i);
        this.capacity = newcapacity;
        this.queue = newqueue;
        this.head = 0;
        this.tail = size;
    }

    @SuppressWarnings("unchecked")
    private T[] newArray(int size) {
        return (T[]) new Object[size];
    }

    public boolean add(T o) {
        queue[tail] = o;
        int newtail = (tail + 1) % capacity;
        if (newtail == head)
            throw new IndexOutOfBoundsException("Queue full");
        tail = newtail;
        return true; // we did add something
    }

    public T remove(int i) {
        if (i != 0)
            throw new IllegalArgumentException("Can only remove head of queue");
        if (head == tail)
            throw new IndexOutOfBoundsException("Queue empty");
        T removed = queue[head];
        queue[head] = null;
        head = (head + 1) % capacity;
        return removed;
    }

    public T get(int i) {
        int size = size();
        if (i < 0 || i >= size) {
            final String msg = "Index " + i + ", queue size " + size;
            throw new IndexOutOfBoundsException(msg);
        }
        int index = (head + i) % capacity;
        return queue[index];
    }

    public int size() {
        // Can't use % here because it's not mod: -3 % 2 is -1, not +1.
        int diff = tail - head;
        if (diff < 0)
            diff += capacity;
        return diff;
    }

    private int capacity;
    private T[] queue;
    private int head;
    private int tail;
}

 size() 计算：

初始化capacity的设定

 判满条件：

tail + 1 == head,(注意：tail表示当前空位，tail + 1当前空位再往后一个，就是整个capacity不填满的)

head > tail 的时候，