#1538. 【2022.09.05 CSP 模拟赛】小树精灵

https://sjzezoj.com/submission/56489

S2OJ/1538/1538.cpp

@@ -0,0 +1,68 @@
+#include <iostream>
+#include <algorithm>
+#include <array>
+#include <vector>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 3005;
+
+int n, root, max_dep;
+long long ans;
+std::vector<int> g[N];
+std::array<int, N> f1, f2, cnt;
+
+void dfs(int u, int f, int d) {
+    cnt[d]++;
+    max_dep = std::max(max_dep, d);
+
+    for (int v : g[u]) {
+        if (v == f) continue;
+
+        dfs(v, u, d + 1);
+    }
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1, x; i <= n; i++) {
+        cin >> x;
+
+        if (x) {
+            g[x].emplace_back(i);
+            g[i].emplace_back(x);
+        } else {
+            root = i;
+        }
+    }
+
+    dfs(root, 0, 1);
+
+    for (int i = 1; i <= n; i++) {
+        std::fill(f1.begin(), f1.end(), 0);
+        std::fill(f2.begin(), f2.end(), 0);
+
+        for (int u : g[i]) {
+            max_dep = 0;
+            std::fill(cnt.begin(), cnt.end(), 0);
+
+            dfs(u, i, 1);
+
+            for (int j = 1; j <= max_dep; j++) {
+                ans += f1[j] * cnt[j];
+                f1[j] += f2[j] * cnt[j];
+                f2[j] += cnt[j];
+            }
+        }
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}