[LeetCode 232] Implement Queue using Stacks

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本文详细介绍了如何仅使用标准堆栈操作实现队列的基本操作：压入、弹出、获取队首元素及判断队列是否为空。通过创建两个堆栈来模拟队列的先进先出（FIFO）特性。

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).

solution:

can use two stacks, queue is FIFO, stack LIFO.

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

    // Removes the element from in front of queue.
    public void pop() {
        shiftStack();
        if(!stackOldest.isEmpty())
            stackOldest.pop();
    }

    // Get the front element.
    public int peek() {
        shiftStack();
        return stackOldest.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stackNewest.isEmpty() && stackOldest.isEmpty();
    }
    
    public void shiftStack() {
        if(stackOldest.isEmpty()){
            while(!stackNewest.isEmpty()){
                stackOldest.push(stackNewest.pop());
            }
        }
    }