栈

定义

---

栈是一种基于后进先出（LIFO）策略的集合类型

Java

---

• 源码（动态调整数组大小版本）

import java.util.Iterator;

public class Stack<Item> implements Iterable<Item> {
    private Item[] a = (Item[]) new Object[1];// 栈元素
    private int N = 0;// 元素数量

    public boolean isEmpty() {
        return N == 0;
    }

    public int size() {
        return N;
    }

    // 动态调整数组大小
    private void resize(int max) {
        // 将栈移动到一个大小为max的新数组
        // 由于某些历史和技术原因，创建泛型数组在Java中是不允许的
        Item[] temp = (Item[]) new Object[max];
        for (int i = 0; i < N; i++)
            temp[i] = a[i];
        a = temp;
    }

    public void push(Item item) {
        // 将元素添加到栈顶
        if (N == a.length) resize(2 * a.length);
        a[N++] = item;
    }

    public Item pop() {
        // 从栈顶删除元素
        Item item = a[--N];
        a[N] = null;// 避免对象游离
        if (N > 0 && N == a.length / 4) resize(a.length / 2);
        return item;
    }

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

    private class ReverseArrayIterator implements Iterator<Item> {
        // 支持后进先出的迭代
        private int i = N;

        public boolean hasNext() {
            return i > 0;
        }

        public Item next() {
            return a[--i];
        }

        public void remove() {
        }
    }
}

• 源码（链表实现版本）

import java.util.Iterator;

public class Stack<Item> implements Iterable<Item> {
    private Node first;// 栈顶（最近添加的元素）
    private int N;// 元素数量

    public boolean isEmpty() {
        return null == first;
    }// 或N == 0

    public int size() {
        return N;
    }

    public void push(Item item) {
        // 向栈顶添加元素
        Node oldFirst = first;
        first = new Node();
        first.item = item;
        first.next = oldFirst;
        N++;
    }

    public Item pop() {
        // 从栈顶删除元素
        Item item = first.item;
        first = first.next;
        N--;
        return item;
    }

    public Iterator<Item> iterator() {
        return null;
    }

    private class Node {
        // 定义了结点的嵌套类
        Item item;
        Node next;
    }

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

        public boolean hasNext() {
            return null != current;
        }

        public Item next() {
            Item item = current.item;
            current = current.next;
            return item;
        }

        public void remove() {
        }
    }
}

• 测试用例

public class Test {
    public static void main(String[] args) {

        String[] ss = {"to", "be", "or", "not", "to", "-", "be", "-", "-", "that", "-", "-", "-", "is"};
        Stack<String> stack = new Stack<>();
        for (String s : ss) {
            if (!s.equals("-")) {
                stack.push(s);
            } else if (!stack.isEmpty()) {
                System.out.print(stack.pop() + " ");
            }
        }
    }
}

• 测试结果

to be not that or be