使用栈实现队列(1)(Java)

class MyQueue
{
    private Stack s1;
    private Stack s2;
    
    public MyQueue(int size)
    {
        this.s1 = new Stack(size);
        this.s2 = new Stack(size);
    }
    
    public boolean isFull()
    {
        return s1.isFull();
    }
    
    public boolean isEmpty()
    {
        return s1.isEmpty();
    }
    
    //时间复杂度: O(1)
    public void EnQueue(int k) throws Exception
    {
        if(s1.isFull())
            throw new Exception("Overflow.");
        else
            s1.push(k);
    }
    
    //时间复杂度: O(n)
    public int DeQueue() throws Exception
    {
        if(s1.isEmpty())
            throw new Exception("Underflow.");
        else
        {
            for(int i = this.s1.getLength(); i > 1; i--)
                s2.push(s1.pop());
            int key = s1.pop();
            while(!s2.isEmpty())
                s1.push(s2.pop());
            return key;
        }
    }
}

class Stack
{
    private int top;
    private int[] a;
    
    public Stack(int size)
    {
        this.top = -1;
        this.a = new int[size];
    }
    
    public boolean isFull()
    {
        return this.top == this.a.length - 1;
    }
    
    public boolean isEmpty()
    {
        return this.top == -1;
    }
    
    public void push(int k) throws Exception
    {
        /*if(this.isFull())
            throw new Exception("Overflow.");*/
        //else
            this.a[++top] = k;
    }
    
    public int pop() throws Exception
    {
        /*if(this.isEmpty())
            throw new Exception("Underflow.");*/
        //else
            return this.a[top--];
    }
    
    public int getLength()
    {
        return this.top + 1;
    }
}