[HAOI2016]食物链

#include<cstdio>#define N 100004int sum[N];struct node{    int v,next;}edge[N*2];int rd[N],cd[N],head[N];int n,m;int read(){    int x=0,f=1;char c=getchar();    while(c>‘9‘||c<‘0‘){if(c==‘-‘)f=-1;c=getchar();}    while(c<=‘9‘&&c>=‘0‘){x=x*10+c-‘0‘;c=getchar();}    return x*f;}int num;int ans;void add_edge(int x,int y){    edge[++num].v=y;edge[num].next=head[x];head[x]=num;}int frist[N],cnt;int dfs(int x){    if(cd[x]==0)    {        sum[x]++;    }    for(int i=head[x];i;i=edge[i].next)    {        if(sum[edge[i].v])sum[x]+=sum[edge[i].v];        else sum[x]+=dfs(edge[i].v);    }    return sum[x];}int main(){    //freopen("chain_2016.in","r",stdin);    //freopen("chain_2016.out","w",stdout);    n=read();m=read();    int a,b;    for(int i=1;i<=m;i++)    {        a=read();b=read();        add_edge(a,b);        cd[a]++;        rd[b]++;    }    for(int i=1;i<=n;i++)    {        if(rd[i]==0&&cd[i]!=0)        {            ans+=dfs(i);        }    }    printf("%d",ans);    return 0;}