1. 栈的要点

1. 栈是一个先入后出(FILO)的有序列表。

2. 栈限制线性表的插入和删除操作只能在一端进行。

3. 允许插入和删除的一端，即变化的一端，称为栈顶；另一段为固定的一端，称为栈底。
   

2. 数组模拟栈

要点：

1. 需要一个指针top用来指示栈顶

2. 当top指向0时，栈拥有第一个元素；故栈空时应该设置top = -1

3. 入栈操作，top++;stack[top] = val;

4. 出栈操作, int val = stack[top]; top–; return val;

5. 但是这个栈不能动态扩展

代码：

package com.ravi.structure.arraystack;

import java.util.Scanner;

public class ArrayStackDemo {
	public static void main(String[] args) {
		ArrayStack arrayStack = new ArrayStack(5);
		String key = "";
		Scanner scanner = new Scanner(System.in);
		boolean loop = true;

		while (loop) {
			System.out.println("show:显示栈");
			System.out.println("exit:退出程序");
			System.out.println("push:入栈");
			System.out.println("pop:出栈");

			key = scanner.next();

			switch (key) {
				case "show":
					arrayStack.list();
					break;
				case "exit":
					scanner.close();
					loop = false;
					break;
				case "push":
					System.out.println("输入一个数");
					int value = scanner.nextInt();
					try {
						arrayStack.push(value);
					}catch (Exception e) {
						System.out.println(e.getMessage());
					}
					break;
				case "pop":
					try {
						int val = arrayStack.pop();
						System.out.println(val + "已经出栈.");
					} catch (Exception exception) {
						System.out.println(exception.getMessage());
					}
					break;
			}
		}
	}
}
class ArrayStack {
	private int maxSize; // 栈的最大大小
	private int[] stack;// 栈数组
	private int top = -1;// 栈顶指针

	public ArrayStack(int maxSize) {
		 this.maxSize = maxSize;
		 stack = new int[maxSize];
	}

	// 判断栈满
	public boolean isFull() {
		return top == maxSize - 1;
	}

	// 判断栈空
	public boolean isEmpty() {
		return top == -1;
	}

	// 入栈
	public void push(int value) {
		if (isFull()) {
			throw new RuntimeException("栈满!");
		}
		top++;
		stack[top] = value;
	}

	// 出栈
	public int pop() {
		if (isEmpty()) {
			throw new RuntimeException("栈空!");
		}
		int value = stack[top];
		top--;
		return value;
	}

	// 遍历栈顶=>栈底
	public void list() {
		if (isEmpty()) {
			for (int i = 0; i < maxSize; i++) {
				System.out.print("| ");
				System.out.print("  ");
				System.out.print(" |");
				System.out.println();
			}
			System.out.println("******");
			return;
		}
		for (int i = maxSize; i > top + 1 ; i--) {
			System.out.print("| ");
			System.out.print("  ");
			System.out.print(" |");
			System.out.println();
		}
		for (int i = top; i > -1 ; i--) {
			System.out.print("| ");
			System.out.print(stack[i]);
			System.out.print(" |");
			System.out.println();
		}
		System.out.println("******");
	}

}

测试：

输出结果：
show:显示栈
exit:退出程序
push:入栈
pop:出栈
push
输入一个数
2
show:显示栈
exit:退出程序
push:入栈
pop:出栈
push
输入一个数
3
show:显示栈
exit:退出程序
push:入栈
pop:出栈
show
|   |
|   |
|   |
| 3 |
| 2 |
******

3. 链表模拟栈

要点：

1. 入栈类似头插法（带头结点的链表）
2. 出栈则是删除第一个结点(头结点是第0个结点)
3. 频繁对第一个节点进行操作的话最好考虑设置一个头结点

代码：

package com.ravi.structure.linkedlistStack;

public class LinkedListStackDemo {
	public static void main(String[] args) {
		LinkedListStack stack = new LinkedListStack();
		stack.add(new Node(6));
		stack.add(new Node(8));
		stack.add(new Node(2));
		stack.list();

		System.out.println("\n" + stack.pop() + "出栈!\n");
		stack.list();
	}
}

class LinkedListStack {
	private final Node head = new Node(-1); // 头结点
	private static int size = 0; // 栈的实际大小

	public void add(Node node) {
		node.next = head.next;
		head.next = node;
		size++;
	}

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

	public int pop() {
		if (isEmpty()) {
			throw new RuntimeException("栈空！不可pop！");
		}
		Node temp = head.next;
		head.next = temp.next;
		return temp.item;
	}

	public void list() {
		if (isEmpty()) {
			System.out.print("| ");
			System.out.print("  ");
			System.out.print(" |");
			System.out.println();
			System.out.println("******");
			return;
		}
		Node p = head.next;
		while (p != null){
			System.out.print("| ");
			System.out.print(p.item);
			System.out.print(" |");
			System.out.println();
			p = p.next;
		}
		System.out.println("******");
	}
}

class Node {
	int item;
	Node next;

	public Node(int item) {
		this.item = item;
		this.next = null;
	}
}

测试：

| 2 |
| 8 |
| 6 |
******

2出栈!

| 8 |
| 6 |
******