use 2 stacks to simulate a queue

class Stack{
private:
    int cur = 0;
    int elem[2000001];
public:
    void push(int n);
    int pop();
    int peek();
    int size();
};

void Stack::push(int n){
    elem[cur] = n;
    cur++;
}

int Stack::pop(){
    cur--;
    return elem[cur + 1];
}

int Stack::peek(){
    return elem[cur - 1];
}

int Stack::size(){
    return cur;
}

class Queue{
private:
    Stack s1, s2;
public:
    void enqueue(int n);
    int dequeue();
    int peek();
    int size();
};

void Queue::enqueue(int n){
    if(s1.size() == 0){
        while(s2.size()){
            s1.push(s2.pop());
        }
    }

    s1.push(n);
}

int Queue::dequeue(){
    if(s2.size() == 0){
        while(s1.size()){
            s2.push(s1.pop());
        }
    }
    return s2.pop();
}

int Queue::peek(){
    if(s2.size() == 0){
        while(s1.size()){
            s2.push(s1.pop());
        }
    }

    return s2.peek();
}

int Queue::size(){
    return s1.size()? s1.size() : s2.size();
}