Sat, 13 Jul 2019 19:20:06 +0800

OnlineJudges/lydsy/4562.cpp

@@ -0,0 +1,36 @@
+#include <bits/stdc++.h>
+using namespace std;
+const int N = 1e5 + 50, M = 2e5 + 50;
+int adj[N], nxt[M], to[M], ecnt;
+int I[N], O[N], F[N];
+inline void addEdge(int f, int t)
+{
+    ecnt++;
+    nxt[ecnt] = adj[f];
+    adj[f] = ecnt;
+    to[ecnt] = t;
+}
+int dfs(int x)
+{
+    if (F[x]) return F[x];
+    int ans = I[x] && !O[x];
+    for (int e = adj[x]; e; e = nxt[e])
+        ans += dfs(to[e]);
+    return F[x] = ans;
+}
+int main()
+{
+    int n, m;
+    scanf("%d%d", &n, &m);
+    for (int i = 0, u, v; i < m; i++)
+    {
+        scanf("%d%d", &u, &v);
+        addEdge(u, v);
+        O[u]++, I[v]++;
+    }
+    int ans = 0;
+    for (int i = 1; i <= n; i++)
+        if (!I[i]) ans += dfs(i);
+    printf("%d", ans);
+    return 0;
+}