https://www.nowcoder.com/practice/54275ddae22f475981afa2244dd448c6?tpId=265&tqId=39213&rp=1&ru=/exam/oj/ta&qru=/exam/oj/ta&sourceUrl=%2Fexam%2Foj%2Fta%3FtpId%3D13&difficulty=undefined&judgeStatus=undefined&tags=&title=

class Solution
{
public:
    void push(int node) {
        stack1.push(node);
    }

    int pop() {
        if(!stack2.empty())
        {
            int val=stack2.top();
            stack2.pop();
            return val;
        }

        while(!stack1.empty())
        {
            int val=stack1.top();
            stack1.pop();
            stack2.push(val);
        }

        return pop();
    }

private:
    stack<int> stack1;
    stack<int> stack2;
};