原题链接：AcWing 129. 火车进栈
参考题解：AcWing 129. 火车进栈-yxc版本

#include <bits/stdc++.h>
using namespace std;
#define ll long long

int n;
vector<int> state1;
stack<int> state2;
int state3=1;
int cnt=20;

void dfs()
{
    if(cnt==0)
        return;
    if(state1.size()==n)
    {
        cnt--;
        for(int i=0;i<n;i++)
        {
            cout<<state1[i];
        }
        cout<<endl;
        return ;
    }
    if(state2.size()>0)
    {
        state1.push_back(state2.top());
        state2.pop();
        dfs();
        state2.push(state1.back());
        state1.pop_back();
    }
    if(state3<=n)
    {
        state2.push(state3++);
        dfs();
        state2.pop();
        state3--;
    }
}

int main()
{
    std::ios::sync_with_stdio(false);
    cin>>n;
    dfs();
    return 0;
}