Leetcode 225，232 用两个队列实现栈，用两个栈实现队列

232 Implement Queue using Stacks My Submissions Question
Total Accepted: 29497 Total Submissions: 86968 Difficulty: Easy
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).**

s1,s2两个栈，出队列时如果s2为空，将s1的元素全部Pop到s2再从s2 Pop。否则直接s2 Pop。
入队列时直接s1 Push

代码

public class Queue {    Stack<int> s1 = new Stack<int>();    Stack<int> s2 = new Stack<int>();    // Push element x to the back of queue.    public void Push(int x) {        s1.Push(x);    }    // Removes the element from front of queue.    public void Pop() {        if(s2.Count==0) {           while(s1.Count>0) s2.Push(s1.Pop());        }        s2.Pop();    }    // Get the front element.    public int Peek() {        if(s2.Count==0) {           while(s1.Count>0) s2.Push(s1.Pop());        }        return s2.Peek();    }    // Return whether the queue is empty.    public bool Empty() {        return s1.Count==0 && s2.Count==0;    }}

**225 Implement Stack using Queues My Submissions Question
Total Accepted: 27669 Total Submissions: 91210 Difficulty: Easy
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).**

感觉这个方法比较笨，不知道有没有更好的。始终保持一个队列为空。入队列入非空的队列。出队列时把非空队列的除最后一个外全部转移到空的队列，然后出非空队列最后一个。

代码

public class Stack {    Queue q1=new Queue(),q2=new Queue();    // Push element x onto stack.    public void Push(int x) {        if(q1.Count!=0) q1.Enqueue(x);        else if(q2.Count!=0) q2.Enqueue(x);        else q1.Enqueue(x);    }    // Removes the element on top of the stack.    public void Pop() {        Queue q = q1.Count==0?q2:q1;        Queue qq = q1.Count==0?q1:q2;        while(q.Count>1) {            qq.Enqueue(q.Dequeue());        }        q.Dequeue();    }    // Get the top element.    public int Top() {        Queue q = q1.Count==0?q2:q1;        Queue qq = q1.Count==0?q1:q2;        while(q.Count>1) {            qq.Enqueue(q.Dequeue());        }        int ret = (int)q.Peek();        qq.Enqueue(q.Dequeue());        return ret;    }    // Return whether the stack is empty.    public bool Empty() {        return q1.Count==0 && q2.Count==0;    }}