队列和栈相互实现

Posted 2022-05-09 *平芜尽处是春山*

力扣225.用队列实现栈

1、使用一个队列

class MyStack 
    Queue<Integer> queue;

    public MyStack() 
        queue = new LinkedList<Integer>();
    
    
    public void push(int x) 
        int n = queue.size();
        queue.offer(x);
        for (int i = 0; i < n; i++) 
            queue.offer(queue.poll());
        
    

    public int pop() 
        return queue.poll();
    

    public int top() 
        return queue.peek();
    
    
    public boolean empty() 
        return queue.isEmpty();
    

运行截图：

2、使用两个队列

public class MyStack 
    private Queue<Integer> queue1=new LinkedList<>();
    private Queue<Integer> queue2=new LinkedList<>();

    public void push(int x) 
        queue2.offer(x);
        while(! queue1.isEmpty())
            queue2.offer(queue1.poll());
        
        Queue<Integer> tmp=queue1;
        queue1=queue2;
        queue2=tmp;
    

    public int pop() 
        return  queue1.poll();
    

    public int top() 
        return queue1.peek();
    

    public boolean empty() 
        return queue1.isEmpty();
    

运行截图：

力扣232.用栈实现队列（使用两个栈）

public class MyQueue 
    private Stack<Integer> s1;
    private Stack<Integer> s2;
    public MyQueue()
        s1=new Stack<>();
        s2=new Stack<>();
    
    public void push(int x)
        if(s1.isEmpty())
            s1.push(x);
        else
            while(! s1.isEmpty())
                s2.push(s1.pop());
            
            s1.push(x);
            while(! s2.isEmpty())
                s1.push(s2.pop());
            
        
    

    public int pop()
        return s1.pop();
    
    public int peek()
        return s1.peek();
    
    public boolean empty()
        return s1.isEmpty();
    

运行截图：