用棧實現隊列（C++）

用棧實現隊列（C++）

準備兩個棧 instack,outstack
入隊時，push到instack棧中
出隊時：
1.如果outstack爲空，把instack棧中元素每一個push到outstack,並從instack中彈出，出隊列即彈出outstack的棧頂元素
2.如果outstack不爲空，彈出outstack棧頂元素

class MyQueue {
private:
	void checkoutStack()
	{
		if (outstack.empty()) //爲空
		{
			while (!instack.empty())
			{
				outstack.push(instack.top());
				instack.pop();
			}
		}
	}
public:

	stack<int> instack;
	stack<int> outstack;

	/** Initialize your data structure here. */
	MyQueue()
	{
		
	}

	/** Push element x to the back of queue. */
	void push(int x)
	{
		instack.push(x);
	}

	/** Removes the element from in front of queue and returns that element. */
	int pop() {
		checkoutStack();

		int tmp = outstack.top();
		outstack.pop();
		return tmp;
	}

	/** Get the front element. */
	int peek()
	{
		checkoutStack();
		return outstack.top();
	}

	/** Returns whether the queue is empty. */
	bool empty() {
		return instack.empty() && outstack.empty();
	}
};