给出N个点，M条无向边，保证现在每个点的度至少为K，让我们找到一个长度大于K+1的环

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Codeforces 263D Cycle in Graph

#include<iostream>
#include<bits/stdc++.h>
using namespace std;
const int maxn=100000*4;
struct node
{
    int v,nxt;
}edge[maxn*4];
int cnt=0;
int head[maxn];
void add_Edge(int u,int v)
{
    edge[cnt].v=v;
    edge[cnt].nxt=head[u];
    head[u]=cnt++;
}
int step=0;
int k,flag=0;
int sx,sy;
int answer[maxn],dfn[maxn];
int dfs(int u)
{
    if(flag)
        return 1;
    dfn[u]=++step;
    //printf("%d %d\n",u,dfn[u]);
    answer[step]=u;
    for(int i=head[u];i!=-1;i=edge[i].nxt)
    {
        int v=edge[i].v;
        //printf("%d\n",v);
        if(dfn[v])
        {
            if(abs(dfn[v]-dfn[u])>=k)
            {
                flag=1;
                sx=u;
                sy=v;
                return 1;
            }
        }
        else
        {
            dfs(v);
        }
    }
}
int main ()
{
    int n,m;
    scanf("%d%d%d",&n,&m,&k);
    cnt=0;
    memset(head,-1,sizeof(head));
    for(int i=1;i<=m;i++)
    {
        int xx,yy;
        scanf("%d%d",&xx,&yy);
        add_Edge(xx,yy);
        add_Edge(yy,xx);
    }
    //printf("111\n");
    dfs(1);
    printf("%d\n",dfn[sy]-dfn[sx]+1);
    for(int i=dfn[sx];i<=dfn[sy];i++)
        printf("%d ",answer[i]);
}