225. Implement Stack using Queues_freeeⅹxⅹ-优快云博客

本文链接：https://blog.youkuaiyun.com/FreeeLinux/article/details/53068970

本文介绍了如何使用标准队列操作来实现栈的基本功能，包括push、pop、top和empty等方法，并提供了两种实现方式：一种使用双端队列(deque)，另一种使用普通队列(queue)。

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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).

实话说，这道题出的有点无聊，我用deque做的，下面还有一种stack的思路。

1.deque

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

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

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

    // Return whether the stack is empty.
    bool empty() {
        return que.empty();
    }
private:
    deque<int> que;
};

2.queue

使用队列时需要注意，queue没有pop_back()的方法，因为它是first in first out, 而stack 是 first in last out，所以要通过把前面的元素挪到新加元素的后面，利用queue的pop方法，每次弹出队列队头元素即可。

代码如下：

class Stack {
public:
	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();
        }
	}

	void pop() {
		que.pop();
	}

	int top() {
		return que.front();
	}

	bool empty() {
		return que.empty();
	}
};