LC.232. Implement Queue using Stacks(use two stacks)

https://leetcode.com/problems/implement-queue-using-stacks/description/
Implement the following operations of a queue using stacks.

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

worse case出现在 所有元素都在stack1 中，第一次 从 stack2 中 pop 

public class LC_232_ImplementQueueUsingStacks {
    private Deque<Integer> stack1;
    private Deque<Integer> stack2;

    /**
     * Initialize your data structure here.
     */
    public LC_232_ImplementQueueUsingStacks() {
        stack1 = new LinkedList<>();
        stack2 = new LinkedList<>();
    }

    /**
     * Push element x to the back of queue.
     */
    public void push(int x) {
        stack1.push(x);
    }

    /**
     * Removes the element from in front of queue and returns that element.
     */
    public int pop() {
        if (!stack2.isEmpty()) {
            return stack2.pop();
        } else {
            shuffle();
            return stack2.pop();
        }
    }

    /**
     * Get the front element.
     */
    public int peek() {
        if (!stack2.isEmpty()) {
            return stack2.peek();
        } else {
            shuffle();
            return stack2.peek();
        }
    }

    private void shuffle() {
        while (!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }
    }

    /**
     * Returns whether the queue is empty.
     */
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}