[LeetCode] Implement Queue using Stacks

脑洞：做到150/216了。面试遇到过这道题了，当时有个follow up question想了很久才答上来，好像是：你现在push是O(1)，pop的worse case是O(n)的，如果要保证pop是O(1)，你要怎么做？（在push不必须是O(1)的情况下）

Problem:

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.

Java Code:

class MyQueue {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();
    
    // Push element x to the back of queue.
    public void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        if (stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.pop());
            }
        }
        stack2.pop();
    }

    // Get the front element.
    public int peek() {
        if (stack2.empty()) {
            while (!stack1.empty()) {
                stack2.push(stack1.pop());
            }
        }
        return stack2.peek();
    }

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