P2863 [USACO06JAN]The Cow Prom S

R66381808

Luogu/P2863/P2863.cpp

@@ -0,0 +1,59 @@
+#include <iostream>
+#include <stack>
+#include <vector>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+const int N = 10005;
+
+int n, m, a, b, ans;
+std::vector<int> g[N];
+
+// Tarjan
+int cnt, dfn[N], low[N];
+int scc_cnt, id[N], siz[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();
+            vis[v] = false;
+            id[v] = scc_cnt;
+            siz[scc_cnt]++;
+        } while (v != u);
+    }
+}
+
+int main() {
+    cin >> n >> m;
+    for (int i = 1; i <= m; i++) {
+        cin >> a >> b;
+        g[a].push_back(b);
+    }
+    for (int i = 1; i <= n; i++) {
+        if (!dfn[i]) tarjan(i);
+    }
+    for (int i = 1; i <= scc_cnt; i++) {
+        if (siz[i] > 1) ans++;
+    }
+    cout << ans << endl;
+    return 0;
+}