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.

class Stack {
private:
    queue <int> q_1;
    queue <int> q_2;
public:
    // Push element x onto stack.
    void push(int x) {
        if(q_1.empty())
          q_1.push(x);
        else
        {
            while(!q_1.empty())
            {
                q_2.push(q_1.front());
                q_1.pop();
            }
            q_1.push(x);
            while(!q_2.empty())
            {
                q_1.push(q_2.front());
                q_2.pop();
            }
        }
    }

    // Removes the element on top of the stack.
    void pop() {
        q_1.pop();
    }

    // Get the top element.
    int top() {
        return q_1.front();
    }

    // Return whether the stack is empty.
    bool empty() {
        return q_1.empty();
    }
};