From 776646995d51a036e9e16923ea0d2382c27873c9 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=BB=84=E6=B5=B7?= <10402852@qq.com>
Date: Wed, 17 Jan 2024 14:24:08 +0800
Subject: [PATCH] 'commit'

---
 TangDou/Topic/HuanGenDp/SubTree.cpp | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/TangDou/Topic/HuanGenDp/SubTree.cpp b/TangDou/Topic/HuanGenDp/SubTree.cpp
index d5bd003..b50562d 100644
--- a/TangDou/Topic/HuanGenDp/SubTree.cpp
+++ b/TangDou/Topic/HuanGenDp/SubTree.cpp
@@ -6,13 +6,18 @@ using namespace std;
 const int N = 100010, M = N << 1;
 int f[N], g[N], mod;
 int pro1[N], pro2[N];
-vector<int> edge[N];
+
+// 链式前向星
+int e[M], h[N], idx, w[M], ne[M];
+void add(int a, int b, int c = 0) {
+    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
+}
 
 void dfs1(int u, int fa) {
     f[u] = 1;
     vector<int> edgeson;
-    for (int i = 0; i < edge[u].size(); i++) {
-        int v = edge[u][i];
+    for (int i = h[u]; ~i; i = ne[i]) {
+        int v = e[i];
         if (v == fa) continue;
         dfs1(v, u);
         f[u] = 1LL * f[u] * (f[v] + 1) % mod;
@@ -37,20 +42,23 @@ void dfs2(int u, int fa) {
         g[u] = 1; // 特判根节点
     else
         g[u] = (g[fa] * (1LL * pro1[u] * pro2[u] % mod) % mod + 1) % mod;
-    for (int i = 0; i < edge[u].size(); i++) {
-        if (edge[u][i] == fa) continue;
-        dfs2(edge[u][i], u);
+    for (int i = h[u]; ~i; i = ne[i]) {
+        int v = e[i];
+        if (v == fa) continue;
+        dfs2(v, u);
     }
 }
 signed main() {
+    // 初始化链式前向星
+    memset(h, -1, sizeof h);
+
     int n;
     cin >> n >> mod;
 
     for (int i = 1; i < n; i++) {
         int a, b;
         cin >> a >> b;
-        edge[a].push_back(b);
-        edge[b].push_back(a);
+        add(a, b), add(b, a);
     }
     dfs1(1, 0);
     dfs2(1, 0);