题解 | 食物链计数

import java.util.*;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        final int N = 100005;
        Scanner in = new Scanner(System.in);
        int n = in.nextInt(), m = in.nextInt();
        List<Integer>[] e = new List[N];
        int []b = new int[N];
        int []d = new int[N];
        int []now = new int[N];
        int []bb = new int[N];
        for(int i = 1;i<=n;i++) {
            e[i] = new ArrayList<>();
            b[i] = 0;
            d[i] = 0;
            now[i] = 0;
            bb[i] = 0;
        }
        for(int i=1;i<=m;i++){
            int u = in.nextInt(), v = in.nextInt();
            add(e,v,u);
            b[u]++;
            bb[u]++;
            d[v]++;
        }
        Queue<Integer> q = new ArrayDeque<>();
        int ans = 0;
        for(int i=1;i<=n;i++) 
            if(b[i] == 0){
                q.offer(i);
                now[i] = 1;
            }
        while(!q.isEmpty()){
            int tmp = q.peek();
            q.poll();
            for(int i:e[tmp]){
                now[i] += now[tmp];
                b[i]--;
                if(b[i] == 0) q.offer(i);
            }
        }
        for(int i=1;i<=n;i++) 
            if(d[i] == 0 && bb[i] != 0)
                ans += now[i];
        System.out.println(ans);
    }
    public static void add(List e[], int x, int y){
        e[x].add(y);
    }
}