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

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 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).

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;
};