Implement Stack using Queues

https://leetcode.com/problems/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).

解题思路：

用了俩queue，思路就很明确了。需要peek或者pop的时候，将一个queue中的元素插入另一个queue，直到只剩一个了，这样才能获得尾端的元素。

也可以只用一个queue，但是在push的时候，就将所有元素立刻反向。

崩溃的是，Queue这个interface，居然没有empty()的方法，连size()的方法都没有。

class MyStack {
    LinkedList<Integer> queue1 = new LinkedList<Integer>();
    LinkedList<Integer> queue2 = new LinkedList<Integer>();
    
    // Push element x onto stack.
    public void push(int x) {
        if(queue1.size() > 0) {
            queue1.offer(x);
        } else {
            queue2.offer(x);
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        if(queue1.size() == 0) {
            LinkedList<Integer> temp = queue1;
            queue1 = queue2;
            queue2 = temp;
        }
        while(queue1.size() > 1) {
            queue2.offer(queue1.poll());
        }
        queue1.poll();
    }

    // Get the top element.
    public int top() {
        if(queue1.size() == 0){
            LinkedList<Integer> temp = queue1;
            queue1 = queue2;
            queue2 = temp;
        }
        while(queue1.size() > 1) {
            queue2.offer(queue1.poll());
        }
        int res = queue1.peek();
        queue2.offer(queue1.poll());
        return res;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue1.size() == 0 && queue2.size() == 0;
    }
}