Java栈和基础队列的实现详解

//基于数组实现的栈
public class MyStack<E> {
    private int size;//数组长度
    //实际存储数据的动态数组
    private List<E> data = new ArrayList<>();
    
    //入栈,默认尾插
    public void push(E val){
        data.add(val);
        size ++;
    }
    
    //弹出栈顶元素，返回栈顶的值
    public E pop(){
        if(isEmpty()){
            //栈为空
            throw new NoSuchElementException("stack is empty!cannot pop!");
        }
        //删元素
       E val =  data.remove(size-1);
        size--;
        return val;
        //上面三句等同于return data.remove(size-1);
    }
    
    //返回栈顶元素，但不出栈
    public E peek(){
        if(isEmpty()){
            throw new NoSuchElementException("stack is empty!cannot peek");
        }
        return data.get(size-1);
    }
    
    // 判断当前栈是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        for (int i = 0; i < size; i++) {
            sb.append(data.get(i));
            if (i != size - 1) {
                // 此时还没到栈顶，还没到数组末尾
                sb.append(", ");
            }
        }
        sb.append("] top");
        return sb.toString();
    }
}
 
//--------------------------
//测试类·
public class StackTest {
    public static void main(String[] args) {
        MyStack<Integer> myStack = new MyStack<>();
        myStack.push(1);
        myStack.push(3);
        myStack.push(5);
        System.out.println(myStack);
        System.out.println(myStack.pop());//删除栈顶
        System.out.println(myStack.peek());//输出栈顶，此时栈顶为3
        System.out.println(myStack.isEmpty());
    }
}

public interface Queue<E> {
    // 入队
    void offer(E val);
    // 出队
    E poll();
    // 返回队首元素
    E peek();
    // 判断队列是否为空
    boolean isEmpty();
}
 
 //基于链表实现的基础队列
public class MyQueue<E> implements Queue<E> {
    // 链表的每个节点
    // ------------------------------
    //内部类
    private class Node {
        E val;
        Node next;
        public Node(E val) {
            this.val = val;
        }
    }
    // ------------------------------
    // 当前队列中的元素个数
    private int size;
    // 队首
    private Node head;
    // 队尾
    private Node tail;
    @Override
    public void offer(E val) {
        Node node = new Node(val);
        if (head == null) {
            // 此时链表为空
            head = tail = node;
        }else {
            tail.next = node;
            tail = node;
        }
        size ++;
    }
 
    @Override
    public E poll() {
        if (isEmpty()) {
            throw new NoSuchElementException("queue is empty!cannot poll!");
        }
        E val = head.val;
        Node node = head;
        head = head.next;
        // 将原来头节点脱钩
        node.next = null;
        size --;
        return val;
    }
 
    @Override
    public E peek() {
        if (isEmpty()) {
            throw new NoSuchElementException("queue is empty!cannot peek!");
        }
        return head.val;
    }
 
    @Override
    public boolean isEmpty() {
        return size == 0;
    }
 
    @Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("front [");
        // 链表的遍历
        for (Node x = head;x != null;x = x.next) {
            sb.append(x.val);
            if (x.next != null) {
                // 还没走到链表尾部
                sb.append(", ");
            }
        }
        sb.append("]");
        return sb.toString();
    }
}
 
//测试类
public class QueueTest {
    public static void main(String[] args) {
        Queue<Integer> queue = new MyQueue<>();
        queue.offer(1);
        queue.offer(3);
        queue.offer(5);
        System.out.println(queue);
    }
}