Implement Stack using Queues

描述

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).

分析

可以用两个队列，q和tmp，q存放元素，tmp用来作中转。

• push(x)，先将x push 到tmp，然后把q中的元素全部弹出来，存入tmp，最后切换q和tmp
• pop()，直接将q的队首元素弹出来即可

该算法push的算法复杂度是O(n), pop的算法复杂度O(1)。

另个一个方法是，让pop是O(n), push是O(1)，思路很类似，就不赘述了。

代码

相关题目

• Implement Queue using Stacks