diff --git a/TangDou/Topic/HuanGenDp/P3478.cpp b/TangDou/Topic/HuanGenDp/P3478.cpp
index 31370ad..37addb1 100644
--- a/TangDou/Topic/HuanGenDp/P3478.cpp
+++ b/TangDou/Topic/HuanGenDp/P3478.cpp
@@ -1,49 +1,53 @@
 #include <bits/stdc++.h>
 using namespace std;
-const int N = 1000010;
+const int N = 1000010, M = N << 1;
 #define int long long
 #define endl "\n"
 
-struct edge {
-    int to, nxt;
-} e[N << 1];
-int n, cnt, id;
-int head[N];
+int n, id;
+// 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++;
+}
+
 int ans;
 int f[N], dep[N], size[N];
-inline void add(int u, int v) {
-    e[++cnt].nxt = head[u];
-    head[u] = cnt;
-    e[cnt].to = v;
-}
-void dfs1(int x, int fa) {
-    size[x] = 1;
-    dep[x] = dep[fa] + 1;
-    for (int i = head[x]; i; i = e[i].nxt) {
-        int y = e[i].to;
-        if (y == fa) continue;
-        dfs1(y, x);
-        size[x] += size[y];
+
+void dfs1(int u, int fa) {
+    size[u] = 1;
+    dep[u] = dep[fa] + 1;
+    for (int i = h[u]; ~i; i = ne[i]) {
+        int v = e[i];
+        if (v == fa) continue;
+        dfs1(v, u);
+        size[u] += size[v];
     }
 }
-void dfs2(int x, int fa) {
-    for (int i = head[x]; i; i = e[i].nxt) {
-        int y = e[i].to;
-        if (y == fa) continue;
-        f[y] = f[x] + n - 2 * size[y];
-        dfs2(y, x);
+void dfs2(int u, int fa) {
+    for (int i = h[u]; ~i; i = ne[i]) {
+        int v = e[i];
+        if (v == fa) continue;
+        f[v] = f[u] + n - 2 * size[v];
+        dfs2(v, u);
     }
 }
 signed main() {
+    memset(h, -1, sizeof h);
     cin >> n;
-    for (int i = 1; i < n; i++) {
-        int u, v;
-        cin >> u >> v;
-        add(u, v), add(v, u);
+    for (int i = 1; i < n; i++) { // n-1条边
+        int a, b;
+        cin >> a >> b;
+        add(a, b), add(b, a); // 换根DP，无向图
     }
+    // 1、第一次dfs,找出：xxx
     dfs1(1, 0);
+
+    // 2、换根
     for (int i = 1; i <= n; i++) f[1] += dep[i];
     dfs2(1, 0);
+
+    // 3、找答案
     for (int i = 1; i <= n; i++)
         if (ans < f[i]) ans = f[i], id = i;
     cout << id << endl;