一起学算法（栈篇）

墨蓝 2023-10-14 11:29 6阅读 0赞

### 1.栈的概念 ###

#### 1.栈的定义 ####

栈是仅限在表尾进行插入和删除的线性表，栈又被称为先进后出的线性表，简称“LIFO”

我们这次用数组作为我们栈的底层数据结构，代码会放到结尾供大家参考使用

#### 2.栈顶的定义 ####

栈是一个线性表，我们允许插入和删除的一端称为栈顶

#### 3.栈底的定义 ####

和栈顶相对的另一端称为栈底，实际上栈底的元素我们不需要关心

#### 4.栈的创建 ####

//数据容器
        private T[] data;
        //容积
        private int capacity;
        //实际存放的元素个数
        private int size;

### 2.入栈 ###

#### 1.入栈的定义 ####

栈元素的插入操作叫做入栈，也可以称为进栈、压栈

#### 2.动画演示 ####

![fd7cc65cd4d043f4c466999a6489f398.gif][]

图中是以链表为底层数据结构的，大家看图了解个大概就可以了，具体可以看代码是怎么实现的

#### 3.代码实现 ####

//在末尾添加元素
        public void addTail(T val) {
            //this.data[size]=val;
            this.add(this.size, val);
        }
    
        //在中间插入元素
        public void add(int index, T val) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            //判断数组是否满
            if(this.size==this.capacity){
                //数组满的话扩容原来数组的两倍
                int newCapacity=this.capacity*2;
                T[] newDate=(T[]) new Object[newCapacity];//创建一个新的数组
                //进行数组迁移
                for (int i = 0; i < this.size; i++) {
                    newDate[i]=this.data[i];
                }
                this.capacity=newCapacity;
                this.data=newDate;
            }

### 3.出栈 ###

#### 1.出栈的定义 ####

栈元素的删除操作叫做出栈，也可称为弹栈

#### 2.动画演示 ####

![5e2385951a6044a56908b972f34a1ad7.gif][]

#### 3.代码演示 ####

//移出末尾
        public T removeTail() {
    //        T val=this.data[size-1];
    //        this.size-=1;
    //        return val;
          return  this.remove(this.size);
        }
       
    
         //中部移出
        public T remove(int index) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            T val = this.data[index];
            for (int i = index; i < this.size; i++) {
                this.data[i] = this.data[i+1];
            }
            this.size -= 1;
    
            if(this.size<=this.capacity/4&&this.capacity/2>1){
                int newCapacity=this.capacity/2;
                T[] newDate=(T[])new Object[newCapacity];
                for (int i = 0; i < this.size; i++) {
                    newDate[i]=this.data[i];
                }
                this.capacity=newCapacity;
                this.data=newDate;
    
            }

### 4.查询栈顶 ###

#### 1.获取栈顶的定义 ####

一般我们可以去查看栈顶的元素是什么

#### 2.动画演示 ####

![85b33ff675a442c6e8037986fd8ebd68.gif][]

#### 3.代码演示 ####

public T peek(T val) {
            return data.getEnByIndex(this.size() - 1);
        }
       
    
      //获得指定索引的元素
        public T getEnByIndex(int index) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            return this.data[index];
        }

基本的栈能用到的就这些基本的方法，接下来给大家一个完整的代码，供大家学习参考

public class StackDemo<T> implements Stack<T> {
        //以我们封装的数组作为栈的底层数据结构
        private  arrSub<T>data;
    
        public StackDemo() {
            data=new arrSub<>();
        }
    
        public StackDemo(int capacity) {
            data=new arrSub<>(capacity);
        }
    
        @Override//弹栈
        public T pop() {
            return data.removeTail();
        }
    
        @Override//往栈中添加元素
        public void push(T val) {
            data.addTail(val);
        }
    
        @Override//查看元素个数
        public int size() {
            return data.getSize();//获得数组元素个数
        }
    
        @Override//判断是否为空
        public boolean isEmpty() {
            return data.isEmpty();
        }
    
        @Override//查看栈顶元素
        public T peek(T val) {
            return data.getEnByIndex(this.size() - 1);
        }
    
        @Override
        public T peek() {
            return null;
        }
    
        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder("栈顶[");
            T[] arr = data.getData();
            for (int i = size() - 1; i >= 0; i--) {
                sb.append(arr[i]);
                if (i != 0) {
                    sb.append(",");
                }
            }
            sb.append("]栈底");
            return sb.toString();
        }
    
    
        public static void main(String[] args) {
            Stack<String> stack = new StackDemo<>(10);
    
            String[] fruits
                    = {"apple", "banana", "peach", "watermelon", "strawberry", "pear", "orange", "grape"};
    
            for (int i = 0; i < fruits.length; i++) {
                stack.push(fruits[i]);
            }
            System.out.println(stack);
    
            while (!stack.isEmpty()) {
                System.out.println("ele-->" + stack.pop());
                System.out.println(stack + "栈中元素的个数：" + stack.size());
            }
        }
    }

public class arrSub<T> {
    
        //数据容器
        private T[] data;
        //容积
        private int capacity;
        //实际存放的元素个数
        private int size;
    
        public arrSub(int capacity) {
            this.capacity = capacity;
            this.data = (T[]) new Object[this.capacity];
            this.size = 0;
        }
    
        public arrSub() {
            this.capacity = 10;
        }
    
        // 判断数据容器是否为空
        public boolean isEmpty() {
            return this.size == 0;
        }
        public T[] getData(){
           return  this.data;
        }
    
        //获得数组的容积
        public int getCapacity() {
            return this.capacity;
        }
    
        //获取元素的个数
        public int getSize() {
            return this.size;
        }
    
        //首行添加元素
        public void addTop(T val) {
    //        for (int i = size-1; i >=0 ; i--) {
    //            this.data[i+1]=this.data[i];
    //        }
    //        this.data[0]=val;
            this.add(0, val);
        }
    
        //在末尾添加元素
        public void addTail(T val) {
            //this.data[size]=val;
            this.add(this.size, val);
        }
    
        //在中间插入元素
        public void add(int index, T val) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            //判断数组是否满
            if(this.size==this.capacity){
                //数组满的话扩容原来数组的两倍
                int newCapacity=this.capacity*2;
                T[] newDate=(T[]) new Object[newCapacity];//创建一个新的数组
                //进行数组迁移
                for (int i = 0; i < this.size; i++) {
                    newDate[i]=this.data[i];
                }
                this.capacity=newCapacity;
                this.data=newDate;
            }
    
    
            for (int i = this.size - 1; i >= index; i--) {
                this.data[i + 1] = this.data[i];
            }
            this.data[index] = val;
            this.size += 1;
        }
    
        //获得指定索引的元素
        public T getEnByIndex(int index) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            return this.data[index];
        }
    
        //获得指定元素的索引
        public int getEnByVal(int index, T val) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            for (int i = 0; i < this.size; i++) {
                if (this.data[i] == val) {
    
                    return i;
                }
            }
            return -1;
        }
    
        //判断是否包含某种布局
        public boolean contains(T val) {
            for (int i = 0; i < this.size; i++) {
                if (this.data[i] == val) {
                    return true;
                }
            }
            return false;
        }
    
        //修改指定索引的元素
        public void upLoad(T var, int index) {
            if (index < 0 || index > this.size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            this.data[index] = var;
        }
        //重写toString的方法
    
        @Override
        public String toString() {
            StringBuilder stringBuilder = new StringBuilder(10);
            for (int i = 0; i < this.size; i++) {
                stringBuilder.append(this.data[i] + ",");
            }
            return stringBuilder.substring(0, stringBuilder.lastIndexOf(",") == -1 ? 0 : stringBuilder.lastIndexOf(","));
        }
    
    
        //移出末尾
        public T removeTail() {
    //        T val=this.data[size-1];
    //        this.size-=1;
    //        return val;
          return  this.remove(this.size);
        }
    
        //移出首部
        public T removeTop() {
    //        T val=this.data[0];
    //        for (int i = 1; i <this.size; i++) {
    //            this.data[i-1]=this.data[i];
    //        }
    //        this.size-=1;
    //        return val;
          return  this.remove(0);
    
        }
    
        //中部移出
        public T remove(int index) {
            if (index < 0 || index > size) {
                throw new IllegalArgumentException("index is invalid!");
            }
            T val = this.data[index];
            for (int i = index; i < this.size; i++) {
                this.data[i] = this.data[i+1];
            }
            this.size -= 1;
    
            if(this.size<=this.capacity/4&&this.capacity/2>1){
                int newCapacity=this.capacity/2;
                T[] newDate=(T[])new Object[newCapacity];
                for (int i = 0; i < this.size; i++) {
                    newDate[i]=this.data[i];
                }
                this.capacity=newCapacity;
                this.data=newDate;
    
            }
    
    
            return val;
    
    
        }
    
    
        public static void main(String[] args) {
            arrSub myArr = new arrSub(50);
            for (int i = 1; i <= 20; i++) {
                myArr.addTail(i + 10);
            }
            System.out.println(myArr);
    //        System.out.println("20?" + myArr.contains(20));
    //        System.out.println("50?" + myArr.contains(50));
            while (!myArr.isEmpty()) {
                myArr.removeTail();
                System.out.println(myArr);
    
            }
    
    
        }

### leetcode题单： ###

[从尾到头打印链表][Link 1]

public int[] reversePrint(ListNode head) {
            LinkedList<Integer> stack = new LinkedList<Integer>();
            while(head != null) {
                stack.addLast(head.val);
                head = head.next;
            }
            int[] res = new int[stack.size()];
            for(int i = 0; i < res.length; i++)
                res[i] = stack.removeLast();
        return res;
        }

[括号的最大嵌套深度][Link 2]

public int maxDepth(String s) {
          int ans = 0, size = 0;
            for (int i = 0; i < s.length(); ++i) {
                char ch = s.charAt(i);
                if (ch == '(') {
                    ++size;
                    ans = Math.max(ans, size);
                } else if (ch == ')') {
                    --size;
                }
            }
            return ans;
        }

[回文链表][Link 3]

public boolean isPalindrome(ListNode head) {
            if(head==null){
                return false;
            }
            StringBuilder s1=new StringBuilder();
            StringBuilder s2=new StringBuilder();
           while(head!=null){
               s1.append(head.val);
               s2.append(head.val);
               head=head.next;
           }
           s2.reverse();
           return s1.toString().equals(s2.toString());}

这里**以数组为底层结构的队列**代码供大家参考

import java.util.Optional;
    
    public class ArrQueue<T> implements Queue<T> {
        // 队列的容器
        CycleArray<T> data;
    
        // 构造函数
        public ArrQueue() {
            data = new CycleArray<>();
        }
    
        public ArrQueue(int capacity) {
            this.data = new CycleArray<>(capacity);
        }
    
        @Override
        public void offer(T ele) {
            data.addTail(ele);
        }
    
        @Override
        public T poll() {
            return data.removeHead();
        }
    
        @Override
        public int size() {
            return data.getSize();
        }
    
        @Override
        public boolean isEmpty() {
            return data.isEmpty();
        }
    
        @Override
        public T peek() {
            Optional<T> optional = data.getFront();
            if (optional.isPresent()) {
                return optional.get();
            }
            return null;
        }
    
        @Override
        public String toString() {
            return data.toString();
        }
    }

import java.util.Optional;
    
    
    /*解决了队列删除时的时间复杂度O(n),循环队列删除时的时间复杂度为O(1)
     * 假设有两个索引,front-->始终指该数组的第一个元素  tail-->始终指向元素插入位置
     * 如果front==tail时,该循环数组为空
     * 如果(tail+1)%capacity=front时,该数组为满状态
     *
     * */
    /**
     * 循环队列的底层容器
     */
    public class CycleArray<T> {
        // 数据容器
        private T[] data;
        // 容积
        private int capacity;
    
        // 实际存放元素的个数
        private int size;
    
        // 声明两个索引  front--队首 tail--队尾
        int front = 0;
        int tail = 0;
        // front === tail  队列是空的  (tail+1)/capacity == front  队列是满的
    
        public CycleArray() {
            this(10);
        }
    
        public CycleArray(int capacity) {
            this.capacity = capacity + 1;//因为要浪费一个空间,所以在这里要多加一个空间
            this.data = (T[]) new Object[this.capacity];// 浪费一个空间
            this.size = 0;
        }
    
        public T[] getData() {
            return this.data;
        }
    
        // 1、队列是否为空
        public boolean isEmpty() {
            return this.front == this.tail;
        }
    
        // 2、获取数组的容积
        public int getCapacity() {
            return this.capacity;
        }
    
        //3、获取实际存放元素的个数
        public int getSize() {
            return this.size;
        }
    
        // 在队列的尾部添加元素（tail指向待插入元素的位置）
        public void addTail(T val) {
            // 当队列已满，要进行扩容
            if ((this.tail + 1) % this.capacity == this.front) {
                // 新的容积
                int newCapacity = 2 * (this.capacity - 1);
                resize(newCapacity);
            }
    
            // 将val插入到队列的尾部
            this.data[this.tail] = val;
            // tail向后移动
            this.tail = (this.tail + 1) % capacity;
            // 更新size的值
            this.size += 1;
        }
    
    
        private void resize(int newCapacity) {
            T[] newData = (T[]) new Object[newCapacity + 1];
            // 迁移数据 this.front
            int cur = this.front;
            int index = 0;
            while (cur != this.tail) {
                newData[index++] = this.data[cur];
                cur = (cur + 1) % this.capacity;
            }
            // 修改属性
            this.capacity = newCapacity + 1;
            this.data = newData;
            this.front = 0;
            this.tail = this.size;
        }
    
        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder();
            int cur = this.front;
            while (cur != this.tail) {
                sb.append(this.data[cur]);
                if ((cur + 1) % this.capacity != this.tail) {
                    sb.append(",");
                }
                cur = (cur + 1) % this.capacity;
            }
            sb.append("[" + this.front + "<-->" + this.tail + "]");
            return sb.toString();
        }
    
    
        //从队列中移除元素
        public T removeHead() {
            // 1、队列是否为空
            if (this.front == this.tail) {
                return null;
            }
            T val = this.data[this.front];
            this.front = (this.front + 1) % this.capacity;
            this.size--;
            // 缩容
            if (this.size <= this.capacity / 4 && this.capacity / 2 > 1) {
                resize(this.capacity / 2);
            }
            return val;
        }
    
        public Optional<T> getFront() {
            if (isEmpty()) {
                return Optional.empty();
            }
            return Optional.of(this.data[this.front]);
        }
    
    
    //     测试
        public static void main(String[] args) {
            CycleArray<Integer> myArr = new CycleArray<>(5);
            for (int i = 1; i <= 15; i++) {
                myArr.addTail(i + 25);
                System.out.println("添加：" + myArr);
                if (i % 3 == 0) {
                    int val = myArr.removeHead();
                    System.out.println("删除： Header是：" + val);
                }
            }
        }

[fd7cc65cd4d043f4c466999a6489f398.gif]: https://img-blog.csdnimg.cn/img_convert/fd7cc65cd4d043f4c466999a6489f398.gif
[5e2385951a6044a56908b972f34a1ad7.gif]: https://img-blog.csdnimg.cn/img_convert/5e2385951a6044a56908b972f34a1ad7.gif
[85b33ff675a442c6e8037986fd8ebd68.gif]: https://img-blog.csdnimg.cn/img_convert/85b33ff675a442c6e8037986fd8ebd68.gif
[Link 1]: https://leetcode.cn/problems/cong-wei-dao-tou-da-yin-lian-biao-lcof/submissions/
[Link 2]: https://leetcode.cn/problems/maximum-nesting-depth-of-the-parentheses/submissions/
[Link 3]: https://leetcode.cn/problems/aMhZSa/