325 lines
5.8 KiB
Markdown
325 lines
5.8 KiB
Markdown
<!-- GFM-TOC -->
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* [栈](#栈)
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* [1. 数组实现](#1-数组实现)
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* [2. 链表实现](#2-链表实现)
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* [队列](#队列)
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<!-- GFM-TOC -->
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# 栈
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```java
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public interface MyStack<Item> extends Iterable<Item> {
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MyStack<Item> push(Item item);
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Item pop() throws Exception;
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boolean isEmpty();
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int size();
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}
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```
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## 1. 数组实现
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```java
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public class ArrayStack<Item> implements MyStack<Item> {
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// 栈元素数组，只能通过转型来创建泛型数组
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private Item[] a = (Item[]) new Object[1];
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// 元素数量
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private int N = 0;
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@Override
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public MyStack<Item> push(Item item) {
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check();
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a[N++] = item;
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return this;
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}
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@Override
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public Item pop() throws Exception {
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if (isEmpty()) {
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throw new Exception("stack is empty");
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}
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Item item = a[--N];
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check();
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// 避免对象游离
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a[N] = null;
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return item;
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}
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private void check() {
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if (N >= a.length) {
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resize(2 * a.length);
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} else if (N > 0 && N <= a.length / 4) {
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resize(a.length / 2);
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}
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}
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/**
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* 调整数组大小，使得栈具有伸缩性
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*/
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private void resize(int size) {
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Item[] tmp = (Item[]) new Object[size];
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for (int i = 0; i < N; i++) {
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tmp[i] = a[i];
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}
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a = tmp;
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}
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@Override
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public boolean isEmpty() {
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return N == 0;
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}
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@Override
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public int size() {
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return N;
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}
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@Override
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public Iterator<Item> iterator() {
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// 返回逆序遍历的迭代器
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return new Iterator<Item>() {
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private int i = N;
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@Override
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public boolean hasNext() {
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return i > 0;
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}
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@Override
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public Item next() {
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return a[--i];
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}
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};
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}
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}
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```
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## 2. 链表实现
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需要使用链表的头插法来实现，因为头插法中最后压入栈的元素在链表的开头，它的 next 指针指向前一个压入栈的元素，在弹出元素时就可以通过 next 指针遍历到前一个压入栈的元素从而让这个元素成为新的栈顶元素。
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```java
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public class ListStack<Item> implements MyStack<Item> {
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private Node top = null;
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private int N = 0;
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private class Node {
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Item item;
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Node next;
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}
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@Override
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public MyStack<Item> push(Item item) {
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Node newTop = new Node();
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newTop.item = item;
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newTop.next = top;
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top = newTop;
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N++;
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return this;
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}
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@Override
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public Item pop() throws Exception {
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if (isEmpty()) {
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throw new Exception("stack is empty");
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}
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Item item = top.item;
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top = top.next;
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N--;
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return item;
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}
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@Override
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public boolean isEmpty() {
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return N == 0;
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}
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@Override
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public int size() {
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return N;
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}
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@Override
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public Iterator<Item> iterator() {
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return new Iterator<Item>() {
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private Node cur = top;
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@Override
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public boolean hasNext() {
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return cur != null;
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}
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@Override
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public Item next() {
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Item item = cur.item;
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cur = cur.next;
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return item;
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}
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};
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}
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}
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```
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# 队列
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下面是队列的链表实现，需要维护 first 和 last 节点指针，分别指向队首和队尾。
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这里需要考虑 first 和 last 指针哪个作为链表的开头。因为出队列操作需要让队首元素的下一个元素成为队首，所以需要容易获取下一个元素，而链表的头部节点的 next 指针指向下一个元素，因此可以让 first 指针链表的开头。
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```java
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public interface MyQueue<Item> extends Iterable<Item> {
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int size();
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boolean isEmpty();
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MyQueue<Item> add(Item item);
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Item remove() throws Exception;
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}
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```
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```java
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public class ListQueue<Item> implements MyQueue<Item> {
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private Node first;
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private Node last;
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int N = 0;
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private class Node {
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Item item;
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Node next;
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}
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@Override
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public boolean isEmpty() {
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return N == 0;
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}
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@Override
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public int size() {
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return N;
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}
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@Override
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public MyQueue<Item> add(Item item) {
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Node newNode = new Node();
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newNode.item = item;
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newNode.next = null;
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if (isEmpty()) {
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last = newNode;
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first = newNode;
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} else {
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last.next = newNode;
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last = newNode;
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}
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N++;
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return this;
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}
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@Override
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public Item remove() throws Exception {
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if (isEmpty()) {
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throw new Exception("queue is empty");
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}
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Node node = first;
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first = first.next;
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N--;
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if (isEmpty()) {
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last = null;
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}
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return node.item;
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}
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@Override
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public Iterator<Item> iterator() {
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return new Iterator<Item>() {
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Node cur = first;
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@Override
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public boolean hasNext() {
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return cur != null;
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}
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@Override
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public Item next() {
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Item item = cur.item;
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cur = cur.next;
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return item;
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}
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};
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}
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}
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```
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</br><div align="center">欢迎关注我的公众号 CyC2018，这里有最核心的高频基础知识面试题，后台回复关键字 📚 “资料” 可领取复习大纲 ，帮你理清多而杂的面试知识点。</div></br>
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