LeetCode: 225. Implement Stack using Queues

Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Notes:

You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题目大意就是用Queue来实现Stack的功能，包括push，pop，top，empty

分析：

Stack：先进后出
Queue：先进先出

可以用两个Queue来轮流存储数据，当执行pop，top等指令时，因为要返回Queue中最后一个element，所以把之前的所有elements压到另外一个空queue中，剩下唯一一个element的时候，实现poll返回。依次交替

注意事项：pop，top等取出行为，要先检查两个queue是不是都为空

class MyStack 
{
    private Queue<Integer> q1 = new LinkedList<>();
    private Queue<Integer> q2 = new LinkedList<>();
    // Push element x onto stack.
    public void push(int x) 
    {
        if (q1.isEmpty())
        {
            q2.offer(x);
        }
        else
        {
            q1.offer(x);
        }
    }

    // Removes the element on top of the stack.
    public void pop() 
    {
        if (q1.isEmpty() && q2.isEmpty())
        {
            return;
        }
        
        if (q1.isEmpty())
        {
            while (q2.size() > 1)
            {
                q1.offer(q2.poll());
            }
            
            q2.poll();
        }
        else
        {
            while (q1.size() > 1)
            {
                q2.offer(q1.poll());
            }
            
            q1.poll();
        }
    }

    // Get the top element.
    public int top() 
    {
        int res = 0;
        if (q1.isEmpty() && q2.isEmpty())
        {
            return res;
        }
        
        if (q1.isEmpty())
        {
            while (q2.size() > 1)
            {
                q1.offer(q2.poll());
            }
            
            res = q2.poll();
            q1.offer(res);
            
            return res;
        }
        else
        {
            while (q1.size() > 1)
            {
                q2.offer(q1.poll());
            }
            
            res = q1.poll();
            q2.offer(res);
            
            
        }
        
        return res;
    }

    // Return whether the stack is empty.
    public boolean empty() 
    {
        return q1.isEmpty() && q2.isEmpty();
    }
}