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).

采用一个queue，每次新插入一个数据，都将它移至列表头，从而实现last in first out的stack。这样，其他操作都和queue保持一致。

push - O(n), pop - O(1)

class MyStack {
    Queue<Integer> q = new LinkedList<Integer>();
    
    // Push element x onto stack.
    public void push(int x) {
        q.add(x);
        for(int i = 0; i < q.size() - 1; i++) {
            q.add(q.remove());
        }
    }

    // Removes the element on top of the stack.
    public void pop() {
        q.remove();
    }

    // Get the top element.
    public int top() {
        return q.peek();
    }

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