P4017 最大食物链计数(拓扑排序)

题目传送门：https://www.luogu.com.cn/problem/P4017

代码：

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<algorithm>
 4 #include<vector>
 5 #include<map>
 6 #include<queue>
 7 #include<set>
 8 #include<cmath>
 9 #include<list>
10 #include<cstring>
11 #include<string>
12 #define ll long long
13 #define ull unsigned long long
14 #define inf 0x3f3f3f3f
15 #define inff 0x7fffffff
16 using namespace std;
17 const int N = 5000 + 10;
18 const int mod = 80112002;
19 
20 vector<int>G[N];
21 int in[N], val[N], out[N];
22 int n, m;
23 
24 void topo() {
25     
26     queue<int>q;
27     for (int i = 1; i <= n; i++) {
28         if (!in[i]) {
29             q.push(i);
30             val[i] = 1;
31         }
32     }
33     while (!q.empty()) {
34         int tmp = q.front();
35         q.pop();
36         for (int i = 0; i < G[tmp].size(); i++) {
37             int to = G[tmp][i];
38             in[to]--;
39             val[to] = (val[tmp] + val[to]) % mod;
40             if (in[to] == 0) q.push(to);
41         }
42     }
43 
44     return;
45 }
46 
47 int main() {
48 
49     //int n, m;
50     cin >> n >> m;
51     for (int i = 1; i <= m; i++) {
52         int u, v;
53         cin >> u >> v;
54         G[u].push_back(v);
55         in[v]++;
56         out[u]++;
57     }
58     int ans = 0;
59     topo();
60     for (int i = 1; i <= n; i++) {
61         if (!out[i]) ans = (ans + val[i]) % mod;
62     }
63     cout << ans << "\n";
64 
65     return 0;
66 }

 

 
 
posted @ 2022-07-24 15:00  Keyzee  阅读(24)  评论(0)  编辑  收藏  举报