https://www.acwing.com/problem/content/131/
每次操作都先操作state2(先出栈)，这样会使得字典序较小

#include<iostream>
#include<stack>
#include<vector>
using namespace std;

int n , cnt = 20;
vector<int> state1;//要出栈的顺序
stack<int> state2;//车站
int state3 = 1;

void dfs()
{
    if(!cnt) return;//最多输出前20种
    if(state1.size() == n)
    {
        cnt --;
        for(auto x : state1) cout << x;
        cout << '\n';
        return;
    }

    if(state2.size())
    {
        state1.push_back(state2.top());
        state2.pop();
        dfs();
        state2.push(state1.back());
        state1.pop_back();
    }

    if(state3 <= n)//state3表示还没进栈的到第几个了
    {
        state2.push(state3);
        state3 ++;
        dfs();
        state3 --;
        state2.pop();
    }
}

int main()
{
    cin >> n;
    dfs();
    return 0;
}