JAVA 利用栈实现队列

利用2个栈来实现队列:

栈A:1,2,3

栈B:

难点;

poll方法

先将栈A弹栈到只剩一个元素,弹出来的元素push到栈B中保存起来.在吧A中的元素保存起来作为返回值;在弹出去;这时在栈A中弹入(栈B中的所有元素一一弹出);返回保存的元素;

peek方法同理

代码实现:

class QueueImplByStack<E> implements Queue<E>{
    private ArrayStack<E> stackA;
    private ArrayStack<E> stackB;

    public QueueImplByStack() {
        stackA = new ArrayStack<>();
        stackB = new ArrayStack<>();
    }

    @Override
    public void offer(E element) {
        stackA.push(element);

    }

    @Override
    public E poll() {
        if (isEmpty()){
            throw new IllegalArgumentException(" queue is null");
        }while (stackA.size() != 1){
            stackB.push(stackA.pop());
        } E ret = stackA.pop();
        while (!stackB.isEmpty()){
            stackA.push(stackB.pop());
        }
        return ret;
    }

    @Override
    public E peek() {
        if (isEmpty()){
            throw new IllegalArgumentException("");
        }while (stackA.size() != 1){
            stackB.push(stackA.pop());
        } E ret = stackA.peek();
        while (stackB.size() == 0){
            stackA.push(stackB.pop());
        }
        return ret;
    }

    @Override
    public boolean isEmpty() {
        return stackA.isEmpty();
    }

    @Override
    public void clear() {
        stackA.clear();

    }

    @Override
    public int size() {
        return stackA.size();
    }

    @Override
    public Iterator<E> iterator() {
        return stackA.iterator();
    }

    @Override
    public String toString() {
        return stackA.toString();
    }
}