2.5 棧

概述

計(jì)算機(jī)科學(xué)中，stack 是一種線性的數(shù)據(jù)結(jié)構(gòu)，只能在其一端添加數(shù)據(jù)和移除數(shù)據(jù)。習(xí)慣來說，這一端稱之為棧頂，另一端不能操作數(shù)據(jù)的稱之為棧底，就如同生活中的一摞書

先提供一個(gè)棧接口

public interface Stack<E> {
    /**
     * 向棧頂壓入元素
     * @param value 待壓入值
     * @return 壓入成功返回 true, 否則返回 false
     */
    boolean push(E value);

    /**
     * 從棧頂彈出元素
     * @return 棧非空返回棧頂元素, 棧為空返回 null
     */
    E pop();

    /**
     * 返回棧頂元素, 不彈出
     * @return 棧非空返回棧頂元素, 棧為空返回 null
     */
    E peek();

    /**
     * 判斷棧是否為空
     * @return 空返回 true, 否則返回 false
     */
    boolean isEmpty();

    /**
     * 判斷棧是否已滿
     * @return 滿返回 true, 否則返回 false
     */
    boolean isFull();
}

鏈表實(shí)現(xiàn)

public class LinkedListStack<E> implements Stack<E>, Iterable<E> {

    private final int capacity;
    private int size;
    private final Node<E> head = new Node<>(null, null);

    public LinkedListStack(int capacity) {
        this.capacity = capacity;
    }

    @Override
    public boolean push(E value) {
        if (isFull()) {
            return false;
        }
        head.next = new Node<>(value, head.next);
        size++;
        return true;
    }

    @Override
    public E pop() {
        if (isEmpty()) {
            return null;
        }
        Node<E> first = head.next;
        head.next = first.next;
        size--;
        return first.value;
    }

    @Override
    public E peek() {
        if (isEmpty()) {
            return null;
        }
        return head.next.value;
    }

    @Override
    public boolean isEmpty() {
        return head.next == null;
    }

    @Override
    public boolean isFull() {
        return size == capacity;
    }

    @Override
    public Iterator<E> iterator() {
        return new Iterator<E>() {
            Node<E> p = head.next;
            @Override
            public boolean hasNext() {
                return p != null;
            }

            @Override
            public E next() {
                E value = p.value;
                p = p.next;
                return value;
            }
        };
    }

    static class Node<E> {
        E value;
        Node<E> next;

        public Node(E value, Node<E> next) {
            this.value = value;
            this.next = next;
        }
    }
}

數(shù)組實(shí)現(xiàn)

public class ArrayStack<E> implements Stack<E>, Iterable<E>{
    private final E[] array;
    private int top = 0;

    @SuppressWarnings("all")
    public ArrayStack(int capacity) {
        this.array = (E[]) new Object[capacity];
    }

    @Override
    public boolean push(E value) {
        if (isFull()) {
            return false;
        }
        array[top++] = value;
        return true;
    }

    @Override
    public E pop() {
        if (isEmpty()) {
            return null;
        }
        return array[--top];
    }

    @Override
    public E peek() {
        if (isEmpty()) {
            return null;
        }
        return array[top-1];
    }

    @Override
    public boolean isEmpty() {
        return top == 0;
    }

    @Override
    public boolean isFull() {
        return top == array.length;
    }

    @Override
    public Iterator<E> iterator() {
        return new Iterator<E>() {
            int p = top;
            @Override
            public boolean hasNext() {
                return p > 0;
            }

            @Override
            public E next() {
                return array[--p];
            }
        };
    }
}

應(yīng)用

模擬如下方法調(diào)用

public static void main(String[] args) {
    System.out.println("main1");
    System.out.println("main2");
    method1();
    method2();
    System.out.println("main3");
}

public static void method1() {
    System.out.println("method1");
    method3();
}

public static void method2() {
    System.out.println("method2");
}

public static void method3() {
    System.out.println("method3");
}

模擬代碼

public class CPU {
    static class Frame {
        int exit;

        public Frame(int exit) {
            this.exit = exit;
        }
    }
    static int pc = 1; // 模擬程序計(jì)數(shù)器 Program counter
    static ArrayStack<Frame> stack = new ArrayStack<>(100); // 模擬方法調(diào)用棧

    public static void main(String[] args) {
        stack.push(new Frame(-1));
        while (!stack.isEmpty()) {
            switch (pc) {
                case 1 -> {
                    System.out.println("main1");
                    pc++;
                }
                case 2 -> {
                    System.out.println("main2");
                    pc++;
                }
                case 3 -> {
                    stack.push(new Frame(pc + 1));
                    pc = 100;
                }
                case 4 -> {
                    stack.push(new Frame(pc + 1));
                    pc = 200;
                }
                case 5 -> {
                    System.out.println("main3");
                    pc = stack.pop().exit;
                }
                case 100 -> {
                    System.out.println("method1");
                    stack.push(new Frame(pc + 1));
                    pc = 300;
                }
                case 101 -> {
                    pc = stack.pop().exit;
                }
                case 200 -> {
                    System.out.println("method2");
                    pc = stack.pop().exit;
                }
                case 300 -> {
                    System.out.println("method3");
                    pc = stack.pop().exit;
                }
            }
        }
    }
}