SCC
再DFS一下
即可

不过为什么我代码在
BZOJ RE
LG+O2 WA
LG AC

诡异诡异

#include <cstdio>int read(){int x=0, f=1;char ch=getchar();while(ch<'0' || ch>'9'){if(ch=='-')f=-f;ch=getchar();}while(ch>='0' && ch<='9'){x=x*10+(ch-'0');ch=getchar();}return x*f;
}const int MAXV=10111;
const int MAXE=50111;int N, M;struct Vert{int FE;bool Vis;int Bel;
} V[MAXV];struct Edge{int x, y, next;bool i;
} E[MAXE<<1];int Ecnt=0;void addE(int a, int b){++Ecnt;E[Ecnt].x=a;E[Ecnt].y=b;E[Ecnt].next=V[a].FE;V[a].FE=Ecnt;E[Ecnt].i=false;++Ecnt;E[Ecnt].x=b;E[Ecnt].y=a;E[Ecnt].next=V[b].FE;V[b].FE=Ecnt;E[Ecnt].i=true;
}void Vis_init(){for(int i=1;i<=N;++i)V[i].Vis=false;
}int Dfn[MAXV], DFN=0;void DFS(int at){V[at].Vis=true;for(int k=V[at].FE, to;k>0;k=E[k].next){if(E[k].i)  continue;to=E[k].y;if(/*!*/V[to].Vis)  continue;DFS(to);}Dfn[++DFN]=at;
}void DFS(int at, int b){V[at].Vis=true;for(int k=V[at].FE, to;k>0;k=E[k].next){if(!E[k].i) continue;to=E[k].y;if(/*!*/V[to].Vis)  continue;DFS(to, b);}V[at].Bel=b;
}int Bcnt=0;void scc(){Vis_init();for(int i=1;i<=N;++i){if(V[i].Vis)    continue;DFS(i);}Vis_init();for(int i=N, j;i>=1;--i){j=Dfn[i];if(V[j].Vis)    continue;DFS(j, ++Bcnt);}
}bool Win=true;
int ANS=0;int main(){N=read();M=read();for(int i=1, a, b;i<=M;++i){a=read();b=read();addE(b, a);}scc();Vis_init();DFS(Dfn[N]);for(int i=1;i<=N;++i){if(!V[i].Vis){Win=false;break;}}if(Win){for(int i=1;i<=N;++i){if(V[i].Bel==1) ++ANS;}printf("%d\n", ANS);}else    puts("0");return 0;
}