[HAOI 2016]食物链

 1 //It is made by Awson on 2017.10.1
 2 #include <set>
 3 #include <map>
 4 #include <cmath>
 5 #include <ctime>
 6 #include <queue>
 7 #include <stack>
 8 #include <vector>
 9 #include <cstdio>
10 #include <string>
11 #include <cstring>
12 #include <cstdlib>
13 #include <iostream>
14 #include <algorithm>
15 #define LL long long
16 #define Max(a, b) ((a) > (b) ? (a) : (b))
17 #define Min(a, b) ((a) < (b) ? (a) : (b))
18 #define sqr(x) ((x)*(x))
19 #define lowbit(x) ((x)&(-(x)))
20 using namespace std;
21 const int N = 100000;
22 const int M = 200000;
23 void read(int &x) {
24     char ch; bool flag = 0;
25     for (ch = getchar(); !isdigit(ch) && ((flag |= (ch == ‘-‘)) || 1); ch = getchar());
26     for (x = 0; isdigit(ch); x = (x<<1)+(x<<3)+ch-48, ch = getchar());
27     x *= 1-2*flag;
28 }
29 
30 int n, m, u, v;
31 struct tt {
32     int to, next;
33 }edge[M+N+5];
34 int path[N+5], top;
35 int in[N+5], out[N+5];
36 int ans[N+5];
37 
38 void add(int u, int v) {
39     edge[++top].to = v;
40     edge[top].next = path[u];
41     path[u] = top;
42 }
43 void topsort() {
44     queue<int>Q;
45     while (!Q.empty()) Q.pop();
46     for (int i = 1; i <= n; i++)
47         if (!in[i]&&out[i]) {
48             Q.push(i);
49             ans[i] = 1;
50         }
51     while (!Q.empty()) {
52         int u = Q.front(); Q.pop();
53         for (int i = path[u]; i; i = edge[i].next) {
54             int v = edge[i].to;
55             in[v]--, ans[v] += ans[u];
56             if (!in[v]) Q.push(v);
57         }
58     }
59 }
60 void work() {
61     read(n), read(m);
62     for (int i = 1; i <= m; i++) {
63         read(u), read(v);
64         add(u, v);
65         in[v]++, out[u]--;
66     }
67     for (int i = 1; i <= n; i++)
68         if (!out[i]&&in[i]) add(i, n+1);
69     topsort();
70     printf("%d\n", ans[n+1]);
71 }
72 int main() {
73     work();
74     return 0;
75 }