225. Implement Stack using Queues 用两个队列实现栈的基本操作。

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 onlypush to back, peek/pop from front, size, andis 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).

解题思路：保证一个队列为空，当保存新元素时，将其保存至空队列，然后将另一个队列的所有元素存入此队列。

我的代码：

class Stack {private:    queue<int> q1,q2;//保证一个队列为空，当保存新元素时，将其保存至空队列，然后将另一个队列的所有元素存入此队列。public:    // Push element x onto stack.    void push(int x) {        if(q1.empty()){            q1.push(x);            while(!q2.empty()){                int tmp=q2.front();                q2.pop();                q1.push(tmp);            }        }        else if(q2.empty()){            q2.push(x);            while(!q1.empty()){                int tmp=q1.front();                q1.pop();                q2.push(tmp);            }        }    }    // Removes the element on top of the stack.    void pop() {        if(!q1.empty())q1.pop();        else q2.pop();    }    // Get the top element.    int top() {        if(!q1.empty())return q1.front();        else return q2.front();    }    // Return whether the stack is empty.    bool empty() {        return q1.empty()&&q2.empty();    }};