P3387 【模板】缩点

R65988384

Luogu/P3387/P3387.cpp

@@ -0,0 +1,98 @@
+#include <cstring>
+#include <iostream>
+#include <queue>
+#include <stack>
+#include <vector>
+
+using std::cin;
+using std::cout;
+using std::endl;
+
+// Limits
+const int N = 10005;
+
+int n, m, u, v, a[N];
+std::vector<int> g[N], g2[N];
+
+// Tarjan
+int cnt, dfn[N], low[N];
+int scc_cnt, id[N];
+int w[N];
+bool vis[N];
+std::stack<int> st;
+
+void tarjan(int u) {
+    dfn[u] = low[u] = ++cnt;
+    vis[u] = true;
+    st.push(u);
+    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;
+            w[scc_cnt] += a[v];
+        } while (v != u);
+    }
+}
+
+// Shortest Path
+int dist[N];
+int spfa(int s) {
+    memset(dist, -0x3f, sizeof(dist));
+    std::queue<int> q;
+    q.push(s);
+    dist[s] = 0;
+    int res = 0;
+    while (!q.empty()) {
+        int u = q.front();
+        q.pop();
+        res = std::max(res, dist[u] + w[u]);
+        for (int v : g2[u]) {
+            if (dist[v] < dist[u] + w[u]) {
+                dist[v] = dist[u] + w[u];
+                q.push(v);
+            }
+        }
+    }
+    return res;
+}
+
+int main() {
+    cin >> n >> m;
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i];
+    }
+    for (int i = 1; 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[i] != id[v]) {
+                g2[id[i]].push_back(id[v]);
+            }
+        }
+    }
+    int ans = 0;
+    for (int i = 1; i <= scc_cnt; i++) {
+        ans = std::max(ans, spfa(i));
+    }
+    cout << ans << endl;
+    return 0;
+}