232. Implement Queue using Stacks

https://leetcode.com/problems/implement-queue-using-stacks/description/
Solution 1. Use 1 stack.  Recursively push x to the bottom of the stack.
    stack<int> st;
public:
    /** Initialize your data structure here. */
    MyQueue() {
     
    }
    /** Push element x to the back of queue. */
    void push(int x) {
        if(st.empty()) {
            st.push(x);
            return;
        }
        int y = st.top();
        st.pop();
        push(x);
        st.push(y);
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        int x = st.top();
        st.pop();
        return x;
    }
    /** Get the front element. */
    int peek() {
        return st.top();
    }
    /** Returns whether the queue is empty. */
    bool empty() {
        return st.empty();
    }

};
Solution 2. Use 2 stacks for input and output, respectively. move from input to output stack only when output stack is empty.
class MyQueue {
    stack<int> sti;
    stack<int> sto;
public:
    /** Initialize your data structure here. */
    MyQueue() {
     
    }
    /** Push element x to the back of queue. */
    void push(int x) {
        sti.push(x);
    }
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        peek();
        int x = sto.top();
        sto.pop();
        return x;
    }
    /** Get the front element. */
    int peek() {
        if(sto.empty()) {
            while(!sti.empty()) {
                sto.push(sti.top());
                sti.pop();
            }
        }
        return sto.top();
    }
    /** Returns whether the queue is empty. */
    bool empty() {
        return sti.empty() && sto.empty();
    }
};