利用队列实现栈

Posted 2022-12-29 moris5013

java中的队列LinkedList是动态数组实现的,这里利用两个队列来实现栈

public class MyStack 

    private Queue<Integer> data;
    private Queue<Integer> help;

    public MyStack() 
        data = new LinkedList<Integer>();
        help = new LinkedList<Integer>();
    

    public void push(int pushInt) 
        data.add(pushInt);
    

    public int peek() 
        if (data.isEmpty()) 
            throw new RuntimeException("Stack is empty!");
        
        while (data.size() != 1) 
            help.add(data.poll());
        
        int res = data.poll();
        // 与poll相比，重新追加到队尾中去
        help.add(res);
        swap();
        return res;
    

    public int pop() 
        if (data.isEmpty()) 
            throw new RuntimeException("Stack is empty!");
        
        while (data.size() > 1) 
            help.add(data.poll());
        
        int res = data.poll();
        swap();
        return res;
    

    private void swap() 
        Queue<Integer> tmp = help;
        help = data;
        data = tmp;