LeetCode 232. Implement Queue using Stacks

class MyQueue {
private:
    stack<int> s1; //the same order of queue
    stack<int> s2;
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        while (!s1.empty()){
            int tmp=s1.top(); s1.pop();
            s2.push(tmp);
        }
        s2.push(x);
        while (!s2.empty()){
            int tmp=s2.top(); s2.pop();
            s1.push(tmp);
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int tmp=s1.top(); s1.pop();
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        return s1.top();
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return s1.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */

class MyQueue {
private:
    stack<int> in;
    stack<int> out;
    int front;
public:
    /** Initialize your data structure here. */
    MyQueue() {

    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        if (in.empty()) front=x;
        in.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        if (out.empty()){ //move from in to out
            while (!in.empty()){
                out.push(in.top()); in.pop();
            }
        }
        int tmp=out.top(); out.pop();
        return tmp;
    }
    
    /** Get the front element. */
    int peek() {
        if (!out.empty()) return out.top();
        else return front;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return in.empty() && out.empty();
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * bool param_4 = obj.empty();
 */