while(!in.empty()) 和 while(in.size())

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

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

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

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