What's Stack ?

Where can I use the stack?

Achieve

Interface

public interface Stack<E> {

    int getSize();
    boolean isEmpty();
// 添加一个元素
    void push(E e);
// 取出一个元素（拿出栈顶元素）
    E pop();
//查看栈顶元素的值
    E peek();
}

ArrayStack

public class ArrayStack<E> implements Stack<E> {

    private Array<E> array;
    // 有容积
    public ArrayStack(int capacity){
        array = new Array<>(capacity);
    }
    // 不用传入容量
    public ArrayStack(){
        array = new Array<>();
    }

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

    @Override
    public boolean isEmpty(){
        return array.isEmpty();
    }
    // ArrayStack独有，所以没有声明在接口里
    public int getCapacity(){
        return array.getCapacity();
    }

    @Override
    public void push(E e){
        array.addLast(e);
    }

    @Override
    public E pop(){
        return array.removeLast();
    }

    @Override
    public E peek(){
        return array.getLast();
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Stack: ");
        res.append('[');
        for(int i = 0 ; i < array.getSize() ; i ++){
            res.append(array.get(i));
            if(i != array.getSize() - 1)
                res.append(", ");
        }
        res.append("] top");
        return res.toString();
    }
}

Test

public class Main {

    public static void main(String[] args) {

        ArrayStack<Integer> stack = new ArrayStack<>();

        for(int i = 0 ; i < 5 ; i ++){
            stack.push(i);
            System.out.println(stack);
        }

        stack.pop();
        System.out.println(stack);
    }
}

Stack: [0] top
Stack: [0, 1] top
Stack: [0, 1, 2] top
Stack: [0, 1, 2, 3] top
Stack: [0, 1, 2, 3, 4] top
Stack: [0, 1, 2, 3] top

Test Example

Queue

public interface Queue<E> {

    int getSize();
    boolean isEmpty();
     // 添加元素
   void enqueue(E e);
    // 推出元素
    E dequeue();
   // 队首
    E getFront();
}

public class ArrayQueue<E> implements Queue<E> {

    private Array<E> array;

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

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

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

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

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

    @Override
    public void enqueue(E e){
        array.addLast(e);
    }

    @Override
    public E dequeue(){
        return array.removeFirst();
    }

    @Override
    public E getFront(){
        return array.getFirst();
    }

    @Override
    public String toString(){
        StringBuilder res = new StringBuilder();
        res.append("Queue: ");
        res.append("front [");
        for(int i = 0 ; i < array.getSize() ; i ++){
            res.append(array.get(i));
            if(i != array.getSize() - 1)
                res.append(", ");
        }
        res.append("] tail");
        return res.toString();
    }

    public static void main(String[] args) {

        ArrayQueue<Integer> queue = new ArrayQueue<>();
        for(int i = 0 ; i < 10 ; i ++){
            queue.enqueue(i);
            System.out.println(queue);
            if(i % 3 == 2){
                queue.dequeue();
                System.out.println(queue);
            }
        }
    }
}

循环队列（解决出队复杂度为n(o)的情况）

public class LoopQueue<E> implements Queue<E> {

    private E[] data;
    private int front, tail;
    private int size;

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

    public LoopQueue(){
        this(10);
    }

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

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

    @Override
    public int getSize(){
        return size;
    }

    @Override
    public void enqueue(E e){
//        扩容操作
        if((tail + 1) % data.length == front)
            resize(getCapacity() * 2);
        data[tail] = e;
        tail = (tail + 1) % data.length;
        size ++;
    }

    private void resize(int newCapacity){
        E[] newData = (E[]) new Object[newCapacity + 1];
        for(int i = 0;i < size; i++){
            newData[i] = data[(1 + front) % data.length];
        }
        data = newData;
        front = 0;
        tail = 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 --;
        if(size == getCapacity() / 4 && getCapacity() / 2 != 0)
            resize(getCapacity() / 2);
        return ret;
    }

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

    @Override
    public String toString(){

        StringBuilder res = new StringBuilder();
        res.append(String.format("Queue: size = %d , capacity = %d\n", size, getCapacity()));
        res.append("front [");
//        该遍历和resize的遍历效果是一样的
        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();
    }
}

数组队列和循环队列的比较

算上测试用例的循环O（N），就是O（n）和O（n2）的比较