Implement Queue using Stacks

Description：

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

Code：

class Queue {
public:
    // Push element x to the back of queue.
    stack<int>in;
    stack<int>out;
    void push(int x) {
        in.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        if (out.empty())
        {
            while (!in.empty())
            {
                out.push(in.top());
                in.pop();
            }
        }
        if (!out.empty())
            out.pop();
    }

    // Get the front element.
    int peek(void) {
        if (out.empty())
        {
            while (!in.empty())
            {
                out.push(in.top());
                in.pop();
            }
        }
        if (!out.empty())
            return out.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return in.empty() && out.empty();
    }
};