leetCode(44):Implement Stack using Queues 分类： leetCode 2015-07-19 19:29 50人阅读 评论(0) 收藏

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).

两个队列，永远有一个为空，插入时，插入到不为空的队列中，如果都为空则任意；弹出时，把不为空队列中的所有元素（最后一个除外）转移到另一队列当中，并将最后一个不插入，直接删除即可。如果两个队列均为空，则说明栈为空。

class Stack {
public:
    // Push element x onto stack.
    void push(int x) {//在非空队列中插入
        if (aqueue.empty())
			bqueue.push(x);
		else
			aqueue.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        if (aqueue.empty())
		{//转移
			while (bqueue.size() != 1)
			{
				int tmp = bqueue.front();
				aqueue.push(tmp);
				bqueue.pop();
			}
			bqueue.pop();
		}
		else
		{
			while (aqueue.size() != 1)
			{
				int tmp = aqueue.front();
				bqueue.push(tmp);
				aqueue.pop();
			}
			aqueue.pop();
		}
    }

    // Get the top element.
    int top() {
        int tmp;
		if (aqueue.empty())
		{			
			while (!bqueue.empty())
			{
				tmp= bqueue.front();
				aqueue.push(tmp);
				bqueue.pop();
			}
		}
		else
		{
			while (!aqueue.empty())
			{
				tmp = aqueue.front();
				bqueue.push(tmp);
				aqueue.pop();
			}
		}
		return tmp;
    }

    // Return whether the stack is empty.
    bool empty() {
        return (aqueue.empty() && bqueue.empty());
    }
private:
    queue<int> aqueue;
    queue<int> bqueue;
};