1174. 受欢迎的牛

https://www.acwing.com/problem/content/submission/code_detail/9575745/

AcWing/1174/1174.cpp

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+#include <iostream>
+#include <stack>
+#include <vector>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+// Limits
+const int N = 10005;
+
+int n, m, a, b;
+std::vector<int> g[N];
+
+// Tarjan
+int cnt, dfn[N], low[N];
+int scc_cnt, id[N], siz[N];
+int dout[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);
+    }
+}
+
+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 <= n; i++) {
+        for (int v : g[i]) {
+            if (id[i] != id[v]) {
+                dout[id[i]]++;
+            }
+        }
+    }
+    int z = 0, sum = 0;
+    for (int i = 1; i <= scc_cnt; i++) {
+        if (!dout[i]) {
+            z++;
+            sum += siz[i];
+            if (z > 1) {
+                sum = 0;
+                break;
+            }
+        }
+    }
+    cout << sum << endl;
+    return 0;
+}