自我完善-栈和队列

数据结构完结了栈和队列的内容

对集合框架中的接口有了更深的认识，并且自主实现了栈和队列，发现了双向链表的强大性，可以用来作为栈和队列处理很多问题。

数组实现栈

import java.util.Arrays;

//实现栈
public class MyStack {

    public int[] elem;
    public int usedSize;
    public static final int DEFAULT_CAPACITY = 5;

    public MyStack(){
        elem = new int[DEFAULT_CAPACITY];
    }

    public void push(int e){
        if(usedSize == elem.length){
            elem = Arrays.copyOf(elem,elem.length*2);
        }
        elem[usedSize] = e;
        usedSize++;
    }

    public int pop(){
        if(empty()){
           return -1;
        }
        int val = elem[usedSize-1];
        elem[usedSize] = 0;
        usedSize--;
        return val;
    }

    public int peek(){
        if(empty()){
            return -1;
        }
        return elem[usedSize-1];
    }

    public int size(){
        return usedSize;
    }

    public boolean empty(){
        return usedSize == 0;
    }
}

链表实现队列

public class MyQueue {

    static class ListNode{
        public ListNode prev;
        public ListNode next;
        int val;

        ListNode(int val){
            this.val = val;
        }
    }

    public ListNode head;
    public ListNode last;

    //尾插
    public void offer(int value){
        ListNode node = new ListNode(value);
        if(isEmpty()){
            this.head = node;
            this.last = node;
        }else {
            last.next = node;
            node.prev = last;
            last = node;
        }
    }

    private boolean isEmpty() {
        if(this.head == null){
            return true;
        }
        return false;
    }

    //头删
    public int poll(){
        if(head == null){
            return -1;
        }
        int val = head.val;
        if(head == last){
            head = null;
            last = null;
        }else {
            head = head.next;
            head.prev = null;
        }
        return val;
    }

    public int peek(){
        if(head == null){
            return -1;
        }
        return this.head.val;
    }

    public int size(){
        ListNode cur = head;
        int count = 0;
        while(cur != null){
            count++;
            cur = cur.next;
        }
        return count;
    }

    public boolean empty(){
        if(head == null){
            return true;
        }
        return false;
    }
}

用栈实现队列

import java.util.LinkedList;
import java.util.Stack;

//用栈实现队列
public class MyQueue {

    public Stack<Integer> stack1;
    public Stack<Integer> stack2;

    public MyQueue() {
        stack1 = new Stack<>();
        stack2 = new Stack<>();
    }

    //    void push(int x) 将元素 x 推到队列的末尾
    public void push(int x) {
        stack1.push(x);
    }

    //    int pop() 从队列的开头移除并返回元素
    public int pop() {
        if(empty()){
            return -1;
        }
        if(stack2.empty()){
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        return stack2.pop();
    }

    //    int peek() 返回队列开头的元素
    public int peek() {
        if(empty()){
            return -1;
        }
        if(stack2.empty()){
            while(!stack1.isEmpty()){
                stack2.push(stack1.pop());
            }
        }
        return stack2.peek();
    }

    //    boolean empty() 如果队列为空，返回 true ；否则，返回 false
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}

用队列实现栈

import java.util.LinkedList;
import java.util.Queue;

//用队列实现栈
public class MyStack {

    public Queue<Integer> queue1;
    public Queue<Integer> queue2;

    public MyStack() {
        queue1 = new LinkedList<>();
        queue2 = new LinkedList<>();
    }
    
    public void push(int x) {
        if(!queue1.isEmpty()){
            queue1.offer(x);
        }else if(!queue2.isEmpty()){
            queue2.offer(x);
        }else {
            queue1.offer(x);
        }
    }
    
    public int pop() {
        if(empty()){
            return -1;
        }
        if(!queue1.isEmpty()){
            int size = queue1.size();
            while(size-1 != 0){
                queue2.offer(queue1.poll());
                size--;
            }
            return queue1.poll();
        }else {
            int size = queue2.size();
            while(size-1 != 0){
                queue1.offer(queue2.poll());
                size--;
            }
            return queue2.poll();
        }
    }
    
    public int top() {
        if(empty()){
            return -1;
        }
        if(!queue1.isEmpty()){
            int size = queue1.size();
            int val = -1;
            while(size != 0){
                val = queue1.poll();
                queue2.offer(val);
                size--;
            }
            return val;
        }else {
            int size = queue2.size();
            int val = -1;
            while(size != 0){
                val = queue2.poll();
                queue2.offer(val);
                size--;
            }
            return val;
        }
    }
    
    public boolean empty() {
        return queue1.isEmpty() && queue2.isEmpty();
    }
}