CSU 1804: 有向无环图（拓扑排序）

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define N 100010
 4 typedef long long LL;
 5 typedef pair<int, LL> P;
 6 const int MOD = 1e9 + 7;
 7 struct Edge {
 8     int v, nxt;
 9 } edge[N];
10 int vis[N], n, m, head[N], tot, in[N];
11 LL a[N], b[N], ans;
12 queue<int> que;
13 
14 void Add(int u, int v) { edge[tot] = (Edge) { v, head[u] }; head[u] = tot++; }
15 
16 void BFS() {
17     while(!que.empty()) que.pop();
18     for(int i = 1; i <= n; i++)
19         if(in[i] == 0) que.push(i);
20     while(!que.empty()) {
21         int u = que.front(); que.pop();
22         for(int i = head[u]; ~i; i = edge[i].nxt) {
23             int v = edge[i].v;
24             ans = (ans + a[u] * b[v] % MOD) % MOD;
25             a[v] = (a[u] + a[v]) % MOD;
26             --in[v];
27             if(in[v] == 0) que.push(v);
28         }
29     }
30 }
31 
32 int main() {
33     while(~scanf("%d%d", &n, &m)) {
34         for(int i = 1; i <= n; i++) scanf("%lld%lld", &a[i], &b[i]);
35         ans = tot = 0;
36         memset(head, -1, sizeof(head));
37         memset(in, 0, sizeof(in));
38         for(int i = 1; i <= m; i++) {
39             int u, v; scanf("%d%d", &u, &v);
40             Add(u, v); in[v]++;
41         }
42         BFS();
43         printf("%lld\n", ans % MOD);
44     }
45     return 0;
46 }