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OI-codes/Luogu/P2002/P2002.cpp
2022-01-01 22:15:47 +08:00

71 lines
1.3 KiB
C++

#include <iostream>
#include <stack>
#include <vector>
using std::cin;
using std::cout;
using std::endl;
const int N = 100005;
std::vector<int> g[N];
// Tarjan
int cnt, dfn[N], low[N];
int scc_cnt, id[N];
int din[N];
std::stack<int> st;
bool vis[N];
void tarjan(int u) {
dfn[u] = low[u] = ++cnt;
st.push(u);
vis[u] = true;
for (int v : g[u]) {
if (!dfn[v]) {
tarjan(v);
low[u] = std::min(low[u], low[v]);
} else if (vis[v]) {
low[u] = std::min(low[u], dfn[v]);
}
}
if (low[u] == dfn[u]) {
scc_cnt++;
int v;
do {
v = st.top();
st.pop();
id[v] = scc_cnt;
vis[v] = false;
} while (v != u);
}
}
int main() {
int n, m;
cin >> n >> m;
for (int i = 1; i <= m; i++) {
int u, v;
cin >> u >> v;
g[u].push_back(v);
}
for (int i = 1; i <= n; i++) {
if (!dfn[i]) {
tarjan(i);
}
}
for (int i = 1; i <= n; i++) {
for (int v : g[i]) {
if (id[i] != id[v]) {
din[id[v]]++;
}
}
}
int ans = 0;
for (int i = 1; i <= scc_cnt; i++) {
if (!din[i]) ans++;
}
cout << ans << endl;
return 0;
}