LeetCode-标签为 stack 225. Implement Stack using Queues

原题

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.

You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

题目分析

用队列模拟栈操作。一个队列很难实现高效的栈操作，所以用两个队列，其中一个队列中保证只有一个元素，另一个维护其他元素。当元素入栈时，第一个队列中的唯一元素被自动弹到第二个中，新来的元素成了唯一的元素；当出栈时，将第二个队列的对尾元素弹到第一个队列中。

除了出栈时间复杂度为O(n)外，其他操作为O(1)。

代码实现

    public class MyStack    {        private Queue<int> _q1 = new Queue<int>();        private Queue<int> _q2 = new Queue<int>();        /** Initialize your data structure here. */        public MyStack()        {        }        /** Push element x onto stack. */        public void Push(int x)        {            if (_q1.Count == 0)                _q1.Enqueue(x);            else            {                int front = _q1.Dequeue();                _q2.Enqueue(front);                _q1.Enqueue(x);            }        }        /** Removes the element on top of the stack and returns that element. */        public int Pop()        {            int q2cnt = _q2.Count;            if (q2cnt > 0)            {                while (q2cnt-- > 1)                {                    int tmp = _q2.Dequeue();                    _q2.Enqueue(tmp);                }                _q1.Enqueue(_q2.Dequeue());            }            return _q1.Dequeue();        }        /** Get the top element. */        public int Top()        {            return _q1.Peek();        }        /** Returns whether the stack is empty. */        public bool Empty()        {            return _q1.Count == 0 && _q2.Count == 0;        }    }