LeetCode -- Implement Queue using Stacks

题目描述：

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).

思路：
本题考查的就是使用两个栈来实现一个队列，主要还是对栈本身的一个熟悉。
1.使用s1和s2两个栈
2.push时，直接入s2
3.pop时，先把s2的item 弹出入s1，再从s1中弹出1个，再将s1中元素逐个入s2
4.peek，类似pop，区别在于只取值但不删除。将s2的每个item弹出入s1，n = s1.pop(),然后将s1元素逐个弹入s2，返回n。
5.empty，判断s2是否为空即可。

实现代码：

public class Queue {
public Queue(){
_s1 = new Stack<int>();
_s2 = new Stack<int>();
}
private Stack<int> _s1 ;
private Stack<int> _s2;
// Push element x to the back of queue.
public void Push(int x) {
_s2.Push(x);
}

// Removes the element from front of queue.
public void Pop() {
while(_s2.Count > 0){
_s1.Push(_s2.Pop());
}
_s1.Pop();
while(_s1.Count > 0)
{
_s2.Push(_s1.Pop());
}
}

// Get the front element.
public int Peek() {
while(_s2.Count > 0){
_s1.Push(_s2.Pop());
}

var n = _s1.Peek();
while(_s1.Count > 0){
_s2.Push(_s1.Pop());
}
return n;
}

// Return whether the queue is empty.
public bool Empty() {
return _s2.Count == 0;
}
}