请问下如何解决这段代码的超时问题？（..

#include <set>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;

const int N=10;
bool vis[N];
set <string> ans;

void dfs(int u,int n,int cnt,string s){
    if(cnt==n){
        ans.insert(s);
        return ;
    }
    for(int i=1;i<=n;i++){
        if(vis[i]) continue;
        char c=i+'0';
        vis[i]=true;
        if(i>u) dfs(i,n,cnt+1,(s+c));
        vis[i]=false;
        if(s[s.size()-1]!='-'){
            dfs(-1,n,cnt,(s+"-"));
        }
    }
}

int main(){
    int n;
    cin>>n;
    for(int i=1;i<=n;i++){
        vis[i]=true;
        char c=(i+'0');
        string s="";
        dfs(i,n,1,s+c);
        vis[i]=false;
    }
    cout<<ans.size()<<endl;
    set <string> ::iterator it;
    for(it=ans.begin();it!=ans.end();it++)
    cout<<*it<<endl;
    return 0;
}