F - Endless Walk

https://atcoder.jp/contests/abc245/submissions/30471110

AtCoder/ABC245/F/F.cpp

@@ -0,0 +1,92 @@
+#include <cstring>
+#include <iostream>
+#include <stack>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 2e5 + 5;
+
+int n, m, t[N], ans;
+std::vector<int> g[N], g2[N];
+
+// Tarjan
+int cnt, dfn[N], low[N];
+int scc_cnt, id[N], siz[N];
+int dout[N], din[N];
+bool vis[N];
+std::stack<int> st;
+
+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 (dfn[u] == low[u]) {
+        scc_cnt++;
+        int v;
+        do {
+            v = st.top();
+            st.pop();
+            vis[v] = false;
+            id[v] = scc_cnt;
+            siz[scc_cnt]++;
+        } while (v != u);
+    }
+}
+
+void dfs(int u) {
+    for (int v : g2[u]) {
+        if (t[v] == -1) dfs(v);
+        if (t[v] == 1 || siz[u] > 1) t[u] = 1;
+    }
+    if (siz[u] > 1) t[u] = 1;
+    if (t[u] == -1) t[u] = 0;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    memset(t, 0xff, sizeof(t));
+
+    cin >> n >> m;
+    for (int i = 1, u, v; i <= m; i++) {
+        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[v] != id[i]) {
+                din[id[v]]++;
+                g2[id[i]].push_back(id[v]);
+            }
+        }
+    }
+
+    for (int i = 1; i <= scc_cnt; i++) {
+        if (!din[i]) dfs(i);
+    }
+
+    for (int i = 1; i <= scc_cnt; i++) {
+        if (t[i] == 1) ans += siz[i];
+    }
+
+    cout << ans << endl;
+    return 0;
+}