Implement Stack using Queues 两个队列实现一个栈

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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 emptyoperations 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.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

class Stack {public://以前是两个栈实现一个队列，现在是队列实现栈//两个队列q1，q2//入栈：q1/q2空，入q1；q1不为空，入q1；q2不为空，入q2；//出栈：转换到另一个队列，留下最后一个元素，用于出栈    // Push element x onto stack.    void push(int x) {                if(q1.empty() && q2.empty())            q1.push(x);        else if(!q1.empty())            q1.push(x);        else if(!q2.empty())            q2.push(x);    }    // Removes the element on top of the stack.    void pop() {        //不用考虑合法性        if(!q1.empty())        {            while(q1.size()>1)            {                int tmp=q1.front();                q2.push(tmp);                q1.pop();            }            q1.pop();//最后个元素弹出        }        else{            while(q2.size()>1)            {                int tmp=q2.front();                q1.push(tmp);                q2.pop();            }            q2.pop();//最后个元素弹出        }    }    // Get the top element.    int top() {        //不用考虑合法性        if(!q1.empty())        {            while(q1.size()>1)            {                int tmp=q1.front();                q2.push(tmp);                q1.pop();            }            int t=q1.front();//最后个元素继续入栈            q2.push(t);            q1.pop();            return t;        }        else{            while(q2.size()>1)            {                int tmp=q2.front();                q1.push(tmp);                q2.pop();            }            int t=q2.front();//最后个元素继续入栈            q1.push(t);            q2.pop();            return t;        }    }    // Return whether the stack is empty.    bool empty() {                return (q1.empty()&&q2.empty());    }        queue<int> q1;    queue<int> q2;};

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