手写动态循环队列：数组实现（四)(java版）

快来打我* 2022-10-02 11:49 214阅读 0赞

队列接口：

public interface Queue<E> {
        //获取队列元素个数
        int getSize();
        boolean isEmpty();
        void enqueue(E e);
        E dequeue();
        E getFront();
    }

动态循环队列实现：

public class LoopQueue<E> implements Queue<E> {
    
        private E[] data;
        private int front, tail;
        private int size;  
                                                
        public LoopQueue(int capacity){
            //为什么加1？因为用front == tail判断对空，用（tail+1）%data.length == front判断队满
            //所以要使用一个空间来判断堆满，使用容量得以不变
            data = (E[])new Object[capacity + 1];
            front = 0;
            tail = 0;
            size = 0;
        }
    
        public LoopQueue(){
            this(10);
        }
    
        public int getCapacity(){
            //实际得使用容量
            return data.length - 1;
        }
    
        @Override
        public boolean isEmpty(){
            return front == tail;
        }
    
        @Override
        public int getSize(){
            //队中存在元素个数
            return size;
        }
    
        @Override
        public void enqueue(E e){
    
            if((tail + 1) % data.length == front)
            //为什么是扩容2倍？加入队列元素是1w个，你觉得扩容几个或几十个合适嘛？只要是几倍即可，最    
          //好不要是常数就好
                resize(getCapacity() * 2);
    
            data[tail] = e;
            tail = (tail + 1) % data.length;
            size ++;
        }
    
        @Override
        public E dequeue(){
    
            if(isEmpty())
                throw new IllegalArgumentException("Cannot dequeue from an empty queue.");
    
            E ret = data[front];
            data[front] = null;
            front = (front + 1) % data.length;
            size --;
            //队列过小，缩容
            if(size == getCapacity() / 4 && getCapacity() / 2 != 0)
                resize(getCapacity() / 2);
            return ret;
        }
    
        @Override
        public E getFront(){
            if(isEmpty())
                throw new IllegalArgumentException("Queue is empty.");
            return data[front];
        }
    
        private void resize(int newCapacity){
            //判断队满要使用一个空间
            E[] newData = (E[])new Object[newCapacity + 1];
            for(int i = 0 ; i < size ; i ++)
                newData[i] = data[(i + front) % data.length];
    
            data = newData;
            front = 0;
            tail = size;
        }
    
        @Override
        public String toString(){
    
            StringBuilder res = new StringBuilder();
            res.append(String.format("Queue: size = %d , capacity = %d\n", size, getCapacity()));
            res.append("front [");
            for(int i = front ; i != tail ; i = (i + 1) % data.length){
                res.append(data[i]);
                if((i + 1) % data.length != tail)
                    res.append(", ");
            }
            res.append("] tail");
            return res.toString();
        }
    }