【CodeVS2822】爱在心中

#include<iostream>
#include<cstdio>
#define N 1000000
using namespace std;
struct edge_node
{
    int to,next;
}e[N],d[N];
//int chudu[100];
int head[N],h[N];
bool instack [N];
int stack [N];
int dfn[N], low[N];
int size[N],belong[N];
int cnt,index,top,scc;
int n,m,u,v;
void tarjan(int u)
{ 
    dfn[u] = low[u] = ++index;
    instack[u] = true;
    stack[++top] = u;
    for (int i = head[u];i;i = e[i].next)
    {
        int v = e[i].to;
        if (instack[v])
        {
            low[u] = min(low[u],dfn[v]);
        }
        if (!dfn[v])
        {
            tarjan(v);
            low[u] = min(low[u],low[v]);
        }
    }
    if (low[u] == dfn[u])
    {
        scc++;
        int v = 0;
        for (;u!=v;)
        {
            v=stack[top--];
            instack[v] = false;
            belong[v] = scc;
            size[scc]++;
        }
    }
}
int main()
{
    scanf("%d%d",&n,&m);
    for (int i=1;i<=m;i++)
    {
        scanf("%d%d",&u,&v);
        e[++cnt].to = v;
        e[cnt].next = head[u];
        head[u] = cnt;
    }
    for (int i=1;i<=n;i++)
        if (!dfn[i])
            tarjan(i);
    int ans1 = 0;
    for (int i=1;i<=scc;i++)
        if (size[i]!=1) ans1++;
    printf("%d\n",ans1);
    cnt = 0;
    for (int i=1;i<=n;i++)
    {
        for (int j = head[i];j;j = e[j].next)
        {
            int v = e[j].to;
            if (belong[i] != belong[v])
            {
                d[++cnt].to = belong[v];
                d[cnt].next = h[belong[i]];
                h[belong[i]] = cnt;
            //    chudu[belong[i]] ++;
            }
        }
    }
//    printf("size:");
//    for (int i=1;i<=scc;i++) printf("%d ",size[i]);printf("\n");
//    printf("chudu:");
//    for (int i=1;i<=scc;i++) printf("%d ",chudu[i]);
    int pd =0;
    for (int i=1;i<=scc;i++)
    {
        if (!h[i] && size[i]!=1)
        {
            if (pd!=0) 
            {
                pd = 0; 
                break;
            }
            pd = i;
        }
    }
    if (pd)
    {
        for (int j=1;j<=n;j++)
            if (belong[j] == pd)
                printf("%d ",j);
    }
    else printf("-1");
}