P2860 [USACO06JAN]Redundant Paths G

https://www.luogu.com.cn/record/77636836

Luogu/P2860/P2860.cpp

@@ -0,0 +1,86 @@
+#include <iostream>
+#include <cstring>
+#include <stack>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 5005,
+          M = 10005 << 1;
+
+int f, r, ans;
+int idx, head[N], edge[N], next[N];
+int cnt, dfn[N], low[N];
+int dcc_cnt, siz[N], id[N], in[N];
+bool bridge[N];
+std::stack<int> st;
+
+void add(int u, int v) {
+    next[idx] = head[u];
+    edge[idx] = v;
+    head[u] = idx++;
+}
+
+void tarjan(int u, int in_edge) {
+    dfn[u] = low[u] = ++cnt;
+    st.push(u);
+
+    for (int i = head[u]; ~i; i = next[i]) {
+        int v = edge[i];
+
+        if (!dfn[v]) {
+            tarjan(v, i);
+            low[u] = std::min(low[u], low[v]);
+
+            if (dfn[u] < low[v]) {
+                bridge[i] = bridge[i ^ 1] = true;
+            }
+        } else if (i != (in_edge ^ 1)) {
+            low[u] = std::min(low[u], dfn[v]);
+        }
+    }
+
+    if (dfn[u] == low[u]) {
+        dcc_cnt++;
+        int v;
+        do {
+            v = st.top();
+            st.pop();
+            id[v] = dcc_cnt;
+            siz[dcc_cnt]++;
+        } while (v != u);
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    memset(head, 0xff, sizeof(head));
+
+    cin >> f >> r;
+
+    for (int i = 1, u, v; i <= r; i++) {
+        cin >> u >> v;
+
+        add(u, v);
+        add(v, u);
+    }
+
+    tarjan(1, -1);
+
+    for (int i = 0; i < idx; i++) {
+        if (bridge[i]) {
+            in[id[edge[i]]]++;
+        }
+    }
+
+    for (int i = 1; i <= dcc_cnt; i++) {
+        if (in[i] == 1) ans++;
+    }
+
+    cout << (ans + 1) / 2 << endl;
+
+    return 0;
+}