Implementing queues with stacks

source ： Power button （LeetCode）
link ：https://leetcode-cn.com/problems/implement-queue-using-stacks

Title Description ：

Please use only two stacks to implement the FIFO queue . The queue should support all operations supported by the general queue （push、pop、peek、empty）：
Realization MyQueue class ：
void push(int x) Put the element x Push to the end of the queue
int pop() Remove from the beginning of the queue and return the element
int peek() Returns the element at the beginning of the queue
boolean empty() If the queue is empty , return true ; otherwise , return false

Input ：

[“MyQueue”, “push”, “push”, “peek”, “pop”, “empty”]
[[], [1], [2], [], [], []]

Output ：

[null, null, null, 1, 1, false]

Their thinking ：

1. No matter what structure the bottom layer is implemented with , A logical data structure that always guarantees data first in first out , That's the queue

2. Create a stack st1 and st2, Join the team directly in the stack st1 Join the team ,

3. When the team , if st2 Not empty , Directly out of the stack is out of the team , if st2 Already empty , Then the stack st1 Import all the data inside st2

4. When getting the team head element , if st2 Not empty , Go to the top of the stack and the element is the team leader , If it is empty , The stack st1 Import all the data inside st2

5. Determines if the queue is empty , The entire queue is empty only when both stacks are empty

class MyQueue {
    
public:
    MyQueue() {
    

    }
    
    void push(int x) {
    
        st1.push(x);
    }
    
    int pop() {
    
        /* stack<int>* emptyS = &st1; stack<int>* nonemptyS = &st2; if(!st1.empty()) { swap(emptyS,nonemptyS); } int top = nonemptyS->top(); nonemptyS->pop(); return top; */
        int retval;

        if(!st2.empty())   
        {
    
            retval = st2.top();
            st2.pop();
            return retval;       
        }
        while(!st1.empty())
        {
    
            st2.push(st1.top());
            st1.pop();
        }
        retval = st2.top();
        st2.pop();
        return retval;
        
    }
    
    int peek() {
    
        /* if(!st1.empty()) { // int top1 = st1.top(); //st1.pop(); st2.push(st1.top()); return st2.top(); } else { // int top2 = st2.top(); // st2.pop(); st1.push(st2.top()); return st1.top(); } */
         int retval;
        if(!st2.empty())
        {
    
            retval = st2.top();
            return retval;
        }
        while(!st1.empty())
        {
    
            st2.push(st1.top());
            st1.pop();
        }
        return st2.top();
    }
    
    bool empty() {
    
        return st1.empty() && st2.empty();
    }
private:
    stack<int> st1;
    stack<int> st2;

};