兩個(gè)棧實(shí)現(xiàn)隊(duì)列：

一個(gè)棧用來(lái)入，一個(gè)棧用來(lái)出

public class StackToQueue<T> {
    private Stack<T> stackin;
    private Stack<T> stackout;
    
    public StackToQueue() {
        stackin = new Stack<T>();
        stackout = new Stack<T>();
    }
    
    public void add(T a) {
        stackin.add(a);
    }
    
    public T poll() {
        if(stackout.isEmpty()) {
            while(!stackin.isEmpty()) {
                stackout.add(stackin.pop());
            }   
        }
        if (stackout.isEmpty()) {  
            throw new RuntimeException("No more element.");  
        }  
        return stackout.pop();

    }
    
}

兩個(gè)隊(duì)列實(shí)現(xiàn)棧：

入棧的時(shí)候正常存入一個(gè)隊(duì)列，出棧的時(shí)候用另一個(gè)隊(duì)列保存除最后一個(gè)元素以外的元素，并將最后一個(gè)元素出隊(duì)。

public class QueueToStack<T> {
    Queue<T> queueA;
    Queue<T> queueB;
    Boolean isA;
    
    public QueueToStack() {
        queueA = new LinkedList<>();
        queueB = new LinkedList<>();
        isA = true;
    }
    
    public void push(T a) {
        if(isA) {
            queueA.offer(a);
        }else {
            queueB.offer(a);
        }
    }
    
    public T pop() {
        if(isA) {
            int size = queueA.size();
            for(int i = 0; i < size - 1; i++) {
                queueB.offer(queueA.poll());
            }
            isA = false;
            return queueA.poll();
        }else {
            int size = queueB.size();
            for(int i = 0; i < size - 1; i++) {
                queueA.offer(queueB.poll());
            }
            isA = true;
            return queueB.poll();
        }
    }

}