Leetcode 225:Implement Stack using Queues

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).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

方法一：

用两个队列实现一个栈：

1.压入时，选择不为空的一个队列压入；

2.弹出或求栈顶元素时，首先将非空队列中的所有元素（除队尾元素）弹出保存到另一个队列，然后将队尾元素弹出即可

注意求栈顶元素时，记得将队尾元素也保存到另一个队列。

//用两个队列实现一个栈：
//1.压入时，选择不为空的一个队列压入；
//2.弹出或求栈顶元素时，首先将非空队列中的所有元素（除队尾元素）弹出保存到另一个队列，然后将队尾元素弹出即可
//注意求栈顶元素时，记得将队尾元素也保存到另一个队列
class Stack {
public:
	// Push element x onto stack.
	queue<int> que1;
	queue<int> que2;

	void push(int x) {
		if (!que2.empty())
		{
			que2.push(x);
		}
		else
		{
			que1.push(x);
		}
	}

	// Removes the element on top of the stack.
	void pop() {
		if (!que2.empty())
		{
			while (que2.size()>1)
			{
				que1.push(que2.front());
				que2.pop();
			}
			que2.pop();
		}
		else
		{
			while (que1.size()>1)
			{
				que2.push(que1.front());
				que1.pop();
			}
			que1.pop();
		}
	}

	// Get the top element.
	int top() {
		int topElement = 0;
		if (!que2.empty())
		{
			while (que2.size()>1)
			{
				que1.push(que2.front());
				que2.pop();
			}
			topElement = que2.front();
			que1.push(que2.front());
			que2.pop();
		}
		else
		{
			while (que1.size()>1)
			{
				que2.push(que1.front());
				que1.pop();
			}
			topElement = que1.front();
			que2.push(que1.front());
			que1.pop();
		}

		return topElement;
	}

	// Return whether the stack is empty.
	bool empty() {
		return 	que1.empty() && que2.empty();
	}
};

方法二：

用一个队列实现一个栈：

只需要在压入的方式稍作变化就行，每次压入一个元素，按照队列先进先出的原则，新压入的元素都在队尾，我们只要将整个队列翻转，使得队尾元素移动至队头即可，这样我们后面弹出和求栈顶元素的操作，就和队列一样处理就行。

//用一个队列实现一个栈
class Stack {
public:
	// Push element x onto stack.
	queue<int> que;
	void push(int x) {
		que.push(x);
		for (int i = 0; i<que.size() - 1; ++i)
		{
			que.push(que.front());
			que.pop();
		}
	}

	// Removes the element on top of the stack.
	void pop() {
		que.pop();
	}

	// Get the top element.
	int top() {
		return que.front();
	}

	// Return whether the stack is empty.
	bool empty() {
		return que.empty();
	}
};