模板，注意有个地方打错好多次了：
for(int j = 0;j < scc[i].size();j++)——是j++不是i++！！！
1 #include<iostream>
2 #include<cstdio>
3 #include<cstring>
4 #include<vector>
5 using namespace std;
6
7 const int Maxn = 100010;
8
9 int ind[Maxn],dfn[Maxn],low[Maxn],stack[Maxn],ins[Maxn],inscc[Maxn];
10 vector<int> scc[Maxn],g[Maxn];
11 int cntscc,cntv,n,m,top;
12
13 void tarjan(int x){
14 dfn[x] = low[x] = ++cntv;
15 ins[x] = 1,stack[++top] = x;
16 for(int i = 0;i < g[x].size();i++){
17 int u = g[x][i];
18 if(!dfn[u])tarjan(u),low[x] = min(low[x],low[u]);
19 else if(ins[u])low[x] = min(low[x],dfn[u]);
20 }
21 if(dfn[x] == low[x]){
22 cntscc++; int y = -1;
23 while(y != x){
24 y = stack[top--];
25 ins[y] = 0;
26 inscc[y] = cntscc;
27 scc[cntscc].push_back(y);
28 }
29 }
30 }
31
32 int main(){
33 cin >> n >> m;
34 for(int i = 1;i <= m;i++){
35 int x,y;
36 cin >> x >> y;
37 g[x].push_back(y);
38 }
39 for(int i = 1;i <= n;i++)if(!dfn[i])tarjan(i);
40 for(int i = 1;i <= cntscc;i++){
41 for(int j = 0;j < scc[i].size();j++){
42 int u = scc[i][j];
43 for(int k = 0;k < g[u].size();k++){
44 int v = g[u][k];
45 if(inscc[v] != i)ind[inscc[v]]++;
46 }
47 }
48 }
49 int ans = 0;
50 for(int i = 1;i <= cntscc;i++)if(!ind[i])ans++;
51 cout << ans;
52 return 0;
53 }