[leetcode] Implement Stack using Queues

题目链接在此。

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).

用2个队列：所有数据放在第一个队列，pop和top操作时要用到第二个队列进行暂时存储，最后第二个队列中的数据还是要按顺序放回第一个队列。

class Stack {
public:
	// Push element x onto stack.
	void push(int x) {
		a.push(x);
	}

	// Removes the element on top of the stack.
	void pop() {
		if (empty()) {
			cout << "The stack is empty!\n";
			return ;
		}

		while (a.size() != 1) {
			int tmp = a.front();
			a.pop();
			b.push(tmp);
		}

		a.pop();

		while (!b.empty()) {
			int tmp = b.front();
			b.pop();
			a.push(tmp);
		}
	}

	// Get the top element.
	int top() {
		if (empty()) {
			cout << "The stack is empty!\n";
			return -1;
		}

		int head;
		while (!a.empty()) {
			head = a.front();
			a.pop();
			b.push(head);
		}

		while (!b.empty()) {
			int tmp = b.front();
			b.pop();
			a.push(tmp);
		}

		return head;
	}

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

private:
	queue<int> a;
	queue<int> b;
};