第八届蓝桥杯国赛 发现环

#include <iostream>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <vector>
#include <algorithm>

using namespace std;
vector<int> s[100001];
int f[100001] = {-1};
int e[100001];
int n,a,b,flag = 0,c;
void get(int k)
{
    while(k != -1)
    {
        e[c ++] = k;
        k = f[k];
    }
}
void dfs(int k)
{
    for(int i = 0;i < s[k].size();i ++)
    {
        if(flag == -1)return;
        if(f[k] == s[k][i])continue;
        if(f[s[k][i]])
        {
            f[s[k][i]] = -1;
            flag = -1;
            get(k);
            return;
        }
        f[s[k][i]] = k;
        dfs(s[k][i]);
        f[s[k][i]] = 0;
    }
}
int main()
{
    scanf("%d",&n);
    for(int i = 0;i < n;i ++)
    {
        scanf("%d%d",&a,&b);
        s[a].push_back(b);
        s[b].push_back(a);
    }
    dfs(1);
    sort(e,e + c);
    if(e)printf("%d",e[0]);
    for(int i = 1;i < c;i ++)
    {
        printf(" %d",e[i]);
    }
}