LeetCode Implement Queue using Stacks

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).

还以为做过了，以前那个事用队列实现栈，这个是用栈实现队列

class Queue {
private:
    stack<int> in;
    stack<int> out;
    void transfer(stack<int>& from, stack<int>& to) {
        while (!from.empty()) {
            to.push(from.top());
            from.pop();
        }
    }
public:
    // Push element x to the back of queue.
    void push(int x) {
        transfer(out, in);
        in.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        transfer(in, out);
        out.pop();
    }

    // Get the front element.
    int peek(void) {
        transfer(in, out);
        return out.top();
    }

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