```
class Queue {
public:
stack<int> s1, s2;
//if s2 is empty,transfer the elements from stack s1 to stack s2,then these elements are in reversed order in s2
//peek means get top element in s2(if s2 is empty, call transfer())
//pop means remove top element in s2(if s2 is empty, call transfer())
void transfer()
{
int x = 0;
if (s2.empty())
{
while (!s1.empty())
{
x= s1.top();
s1.pop();
s2.push(x);
}
}
}
// Push element x to the back of queue.
void push(int x) {
s1.push(x);
}
// Removes the element from in front of queue.
void pop(void) {
transfer();
s2.pop();
}
// Get the front element.
int peek(void) {
transfer();
return s2.top();
}
// Return whether the queue is empty.
bool empty(void) {
return s2.empty() && s1.empty();
}
};
```