洛谷P4017 最大食物链计数

拓扑排序板子题

 

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
//Mystery_Sky
//
#define maxn 1000010
#define maxm 5000050
#define mod 80112002
struct Road{
    int next;
    int to;
};
Road road[maxn];
int head[maxm], cnt;
inline void add_edge(int u, int v) {
    road[++cnt].to = v;
    road[cnt].next = head[u];
    head[u] = cnt;
}

int ind[maxn], eat[maxn];
int d[maxn];
int n, m;
queue <int> q;

inline void topo() {
    for(int i = 1; i <= n; i++) {
        if(!ind[i]) {
            q.push(i);
            d[i]++;
        }
    }
    
    while (!q.empty()) {
        int now = q.front();
        q.pop();
        for(int i = head[now]; i; i = road[i].next) {
            int e = road[i].to;
            d[e] = (d[e] + d[now]) % mod;
            if(! --ind[e]) q.push(e);
        }
    }
}

int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= m; i++) {
        int a, b;
        scanf("%d%d", &a, &b);
        add_edge(a, b);
        ind[b]++;
        eat[a]++;
    }
    topo();
    int ans = 0;
    for(int i = 1; i <= n; i++) {
        if(!eat[i]) ans = (ans + d[i]) % mod;
    }
    printf("%d\n", ans);
    return 0;
}