栈:LIFO(后进先出),自己实现一个栈,要求这个栈具有push()
、pop()
(返回栈顶元素并出栈)、peek()
(返回栈顶元素不出栈)、isEmpty()
这些基本的方法。
一、采用数组实现栈
提示:每次入栈之前先判断栈的容量是否够用,如果不够用就用Arrays.copyOf()
进行扩容
import java.util.Arrays;
/**
* 数组实现栈
* @param <T>
*/
class Mystack1<T> {
//实现栈的数组
private Object[] stack;
//数组大小
private int size;
Mystack1() {
stack = new Object[10];//初始容量为10
}
//判断是否为空
public boolean isEmpty() {
return size == 0;
}
//返回栈顶元素
public T peek() {
T t = null;
if (size > 0)
t = (T) stack[size - 1];
return t;
}
public void push(T t) {
expandCapacity(size + 1);
stack[size] = t;
size++;
}
//出栈
public T pop() {
T t = peek();
if (size > 0) {
stack[size - 1] = null;
size--;
}
return t;
}
//扩大容量
public void expandCapacity(int size) {
int len = stack.length;
if (size > len) {
size = size * 3 / 2 + 1;//每次扩大50%
stack = Arrays.copyOf(stack, size);
}
}
}
public class ArrayStack {
public static void main(String[] args) {
Mystack1<String> stack = new Mystack1<>();
System.out.println(stack.peek());
System.out.println(stack.isEmpty());
stack.push("java");
stack.push("is");
stack.push("beautiful");
stack.push("language");
System.out.println(stack.pop());
System.out.println(stack.isEmpty());
System.out.println(stack.peek());
}
}
二、采用链表实现栈
/**
* 链表实现栈
*
* @param <T>
*/
class Mystack2<T> {
//定义链表
class Node<T> {
private T t;
private Node next;
}
private Node<T> head;
//构造函数初始化头指针
Mystack2() {
head = null;
}
//入栈
public void push(T t) {
if (t == null) {
throw new NullPointerException("参数不能为空");
}
if (head == null) {
head = new Node<T>();
head.t = t;
head.next = null;
} else {
Node<T> temp = head;
head = new Node<>();
head.t = t;
head.next = temp;
}
}
//出栈
public T pop() {
T t = head.t;
head = head.next;
return t;
}
//栈顶元素
public T peek() {
T t = head.t;
return t;
}
//栈空
public boolean isEmpty() {
if (head == null)
return true;
else
return false;
}
}
public class LinkStack {
public static void main(String[] args) {
Mystack2 stack = new Mystack2();
System.out.println(stack.isEmpty());
stack.push("Java");
stack.push("is");
stack.push("beautiful");
System.out.println(stack.peek());
System.out.println(stack.peek());
System.out.println(stack.pop());
System.out.println(stack.pop());
System.out.println(stack.isEmpty());
System.out.println(stack.pop());
System.out.println(stack.isEmpty());
}
}
三、采用LinkedList实现栈
- push-----addFirst()
- pop-------removeFirst()
- peek-----getFirst()
- isEmpty-isEmpty()
import java.util.LinkedList;
/**
* LinkedList实现栈
*
* @param <T>
*/
class ListStack<T> {
private LinkedList<T> ll = new LinkedList<>();
//入栈
public void push(T t) {
ll.addFirst(t);
}
//出栈
public T pop() {
return ll.removeFirst();
}
//栈顶元素
public T peek() {
T t = null;
//直接取元素会报异常,需要先判断是否为空
if (!ll.isEmpty())
t = ll.getFirst();
return t;
}
//栈空
public boolean isEmpty() {
return ll.isEmpty();
}
}
public class LinkedListStack {
public static void main(String[] args) {
ListStack<String> stack = new ListStack();
System.out.println(stack.isEmpty());
System.out.println(stack.peek());
stack.push("java");
stack.push("is");
stack.push("beautiful");
System.out.println(stack.peek());
System.out.println(stack.pop());
System.out.println(stack.isEmpty());
System.out.println(stack.peek());
}
}