Implementing Queue using Stacks

 1 class Queue {
 2 private:
 3     stack<int> in;
 4     stack<int> out;
 5     void move(){//move all elements in the IN stack to the OUT stack
 6         while(!in.empty()){
 7             out.push(in.top());
 8             in.pop();
 9         }
10     }
11 public:
12     // Push element x to the back of queue.
13     void push(int x) {
14         in.push(x);
15     }
16 
17     // Removes the element from in front of queue.
18     void pop(void) {//be careful about the condition
19         if(out.empty()) move(); //if the out stack is empty, then move all elements from IN stack to OUT stack
20         if(!out.empty()) out.pop();//if the out stack is not empty, then pop out the top element
21     }
22 
23     // Get the front element.
24     int peek(void) {
25         if(out.empty()) move();
26         if(!out.empty()) return out.top();
27     }
28 
29     // Return whether the queue is empty.
30     bool empty(void) {
31         return in.empty() && out.empty();
32     }
33 };