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.
You must use only standard operations of a queue – which means only
push to back,
peek/pop from front,
is emptyoperations 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).
两种方法，看是需要牺牲 pop 还是 push 的时间复杂度
可不可以牺牲空间，让 pop 和 push 都是 常数时间呢？好像没有？？