P3183 [HAOI2016]食物链

/*
    食物链 拓扑图DP 
*/
#include <cmath>
#include <queue>
#include <cstdio>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>
#define ll long long
using namespace std;

const int A = 1e7+10;
const int B = 1e6+10;
const int mod = 1e9 + 7;
const int inf = 0x3f3f3f3f;

inline int read() {
  char c = getchar();
  int x = 0, f = 1;
  for ( ; !isdigit(c); c = getchar()) if (c == '-') f = -1;
  for ( ; isdigit(c); c = getchar()) x = x * 10 + (c ^ 48);
  return x * f;
}

struct node{int v,nxt;} e[B];

int rd[B],cd[B],n,m,f[B],cnt,head[B];

void modify(int u,int v)
{
    e[++cnt].nxt=head[u];
    e[cnt].v=v;
    head[u]=cnt;
}

queue<int>q;

int main()
{
    int x,y;
    n=read(),m=read();
    for (int i=1;i<=m;i++)
    {
        x=read(),y=read();
        modify(x,y);
        rd[y]++, cd[x]++;
    }
        
    for (int i=1;i<=n;i++)
        if (!rd[i]) 
        {
            if (cd[i]) f[i]=1;
            q.push(i);
        }
        
    
    while (!q.empty())
    {
        //cout<<1<<endl; 
        int u=q.front(); 
        q.pop();
        for (int i=head[u];i;i=e[i].nxt)
        {
            int v=e[i].v;
            f[v]+=f[u];
            rd[v]--;
            if (!rd[v]) q.push(v);
        }
    }
    
    int ans=0;
    for (int i=1;i<=n;i++) if (!cd[i]) ans+=f[i];
    printf("%d",ans); 
}