回家 （无向图割点）

思路：
一是要是割点，而是要分开1和n。
我们通过判断一个割点的儿子能不能到达n，因为割点的儿子跟1是不相连的（从1开始的dfs）

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <stack>
#define LL long long
#define N 200010
#define M 400010
using namespace std;

int n, m, idc = 0, idx = 0;
int head[N], vis[N], exi[N];
int dfn[N], low[N], iscut[N], ans[N];

inline int read(){
    int x = 0, f = 1; char ch = getchar();
    while(ch < '0' || ch > '9'){ if(ch == '-') f = -1; ch = getchar(); }
    while(ch >= '0' && ch <= '9'){ x = x * 10 + ch - '0'; ch = getchar(); }
    return x * f;
}

struct Edge{
    int to, nxt;
}ed[M << 1];

inline void adde(int u, int v){
    ed[++idc].to = v;
    ed[idc].nxt = head[u];
    head[u] = idc;
}

inline int dfs(int u, int f){
    if(vis[u] == 1) return exi[u];
    vis[u] = 1;
    if(u == n) {
        vis[u] = 1; exi[u] = 1;
        return 1;
    }
    for(register int k = head[u]; k; k = ed[k].nxt){
        int v = ed[k].to;
        if(v == f || v == u) continue;
        if( dfs(v, u) ){
            exi[u] = 1;
        }
    }
    return exi[u];
}

stack <int> state;

inline void tarjan(int u, int fa){
    dfn[u] = low[u] = ++idx;
    vis[u] = 1;
    if(u == n) exi[u] = 1;
    state.push(u);
    for(register int i=head[u]; i; i=ed[i].nxt){
        int v = ed[i].to;
        if( !vis[v] ){
            tarjan(v, u);
            exi[u] |= exi[v];
            low[u] = min(low[u], low[v]);
            if(low[v] >= dfn[u] && exi[v]){/*用儿子判*/
                iscut[u] = 1;//u是割点 
            }
        }
        else if( dfn[v] < dfn[u] && v != fa ){ 
            low[u] = min(low[u], dfn[v]);
        }
    }
}

inline void init(){
    memset(head, 0, sizeof(head));
    memset(ed, 0, sizeof(ed));
    memset(vis, 0, sizeof(vis));
    memset(exi, 0, sizeof(exi));
    memset(dfn, 0, sizeof(dfn));
    memset(low, 0, sizeof(low));
    memset(iscut, 0, sizeof(iscut));
    idc = 0; idx = 0;
}

int main(){
    freopen ("home.in", "r", stdin);
    freopen ("home.out", "w", stdout);
    int T; scanf("%d", &T);
    while ( T-- ){
        init();
        scanf ("%d%d", &n, &m);
        for(register int i=1; i<=m; i++){
            int u = read(), v = read();
            adde(u, v); adde(v, u);
        }
        tarjan( 1, 1 );
        int cc = 0;
        for(register int i=2; i<n; i++){
            if( iscut[i] == 1 ) ans[++cc] = i;
        }
        printf("%d\n", cc);
        for(register int i=1; i<cc; i++) printf("%d ", ans[i]);
        if( cc ) printf("%d\n", ans[cc]);
        else printf("\n");/**/
    }
}