洛谷 P4017. 最大食物链计数
原题链接
中等
作者:
我是java同学
,
2024-01-28 11:42:01
,
所有人可见
,
阅读 36
#include <bits/stdc++.h>
using namespace std;
const int N = 5010, M = 500010, mod = 80112002;
int n, m;
int d[N], eat[N], f[N];
int h[N], e[M], ne[M], idx;
queue<int> q;
void add(int a, int b) {
e[idx] = b, ne[idx] = h[a], h[a] = idx ++ ;
}
void topsort() {
for (int i = 1; i <= n; i ++ )
if (!d[i]) {
q.push(i);
f[i] ++ ;
}
while (q.size()) {
auto t = q.front();
q.pop();
for (int i = h[t]; ~i; i = ne[i]) {
int j = e[i];
f[j] = (f[j] + f[t]) % mod;
if (-- d[j] == 0) q.push(j);
}
}
}
int main() {
scanf("%d%d", &n, &m);
memset(h, -1, sizeof h);
while (m -- ) {
int a, b;
scanf("%d%d", &a, &b);
add(a, b);
d[b] ++ , eat[a] ++ ;
}
topsort();
int res = 0;
for (int i = 1; i <= n; i ++ )
if (!eat[i])
res = (res + f[i]) % mod;
cout << res << endl;
return 0;
}